Hello and thanks for reaching out to ask extension. 

Here's some info on watering your cactus since you mention watering from the bottom of the pot:

• Water. Another consideration is water. In nature, Schlumbergera grows in locations with perfect drainage, hanging from tree branches or rocky cliffs. In the rain forest, they receive frequent rains during the wet season, but the water runs off rapidly so the roots do not experience waterlogged conditions. Even in the dry season, there may be 2 to 3 inches of rainfall monthly. Frequent waterings are good if in a very well drained potting mix. In a heavier mix that tends to retain moisture, allow the media to dry somewhat between waterings. Some growers recommend slightly reducing the watering regimen in August and September to harden the stems that will bear the flowers and resuming normal watering once the flower buds start to swell.

And since you mention that the plant is in a south window:

• Sunlight. Similarly, Schlumbergera does not tolerate prolonged periods of strong, direct sunlight. They prefer bright, indirect light, or the light shade of a lath house or shade house. Some people have great success by putting them outdoors during the frost-free growing period, such as hanging them in their pots in the branches of a shade tree. Avoid the deep shade found under large mature trees such as Norway maple. A shaded porch is another ideal location. Southern exposure is best as long as they are not in direct sun for more than a few hours in the morning. Indoors, they grow well in a bright window, but fresh air movement helps keep plants healthy.

For overall care please see:

https://hort.extension.wisc.edu/articles/holiday-cactus/

Saving an Old Holiday Cactus – A Special Challenge

A really old, large holiday cactus in full bloom is a beautiful marvel, with hundreds of brightly colored flowers open at the same time. It is not unusual to see plants in 12 to 15 inch diameter pots, with a canopy spread of nearly three feet. These plants can be decades old and literally passed down from generation to generation. Such plants may have great sentimental value in addition to their seasonal beauty.

All potted plants need occasional transplanting for various reasons. Most commonly, the plant outgrows its pot. But also note that potting media can become altered over time to no longer be acceptable growing media. As plants get bigger the repotting process becomes more difficult to deal with and avoidance of the inevitable (i.e., procrastination) rears its ugly head. But potting media can eventually become both chemically toxic and physically lethal to plant roots.

As a succulent plant, the stems of holiday cacti can survive for lengthy periods without water if the roots are dead. Entire plant death from slow starvation and dehydration may literally take years. The tell-tale signs of stress include (1) reduced flower bud formation, (2) flower bud abortion, (3) reduced new stem growth, (4) shriveling of the younger stems even when well-watered, (5) “self-pruning” or dropping of younger stem segments and (6) development of small, hair-like adventitious roots at the bases of many stem segments. Another sign of possible root loss is abnormally prolonged moisture in the soil because of the lack of healthy roots to extract soil moisture during normal transpiration.

As with many maladies, prevention is the best cure. Routinely repot your plant at least every 4 to 5 years. In the process, be sure to remove most of the old soil and then inspect the roots to be certain they look healthy. Use a well-drained soil mix and cut back on normal watering for a month or so to allow the development of new feeder roots. Large plants may need to be staked until new roots provide firm anchorage.

Another worthwhile practice is to root a few new cuttings every 3 to 5 years and keep a few healthy young plants on hand in case the original plant succumbs.

When dealing with an obviously stressed plant, unpot it and examine the root system. Cut back any dead or diseased roots and allow the pruned roots to air dry for a few days before repotting. In extreme circumstances, the entire root system may be dead all the way back to the base of the plant. Such plants can be very difficult to save. If this is the case, carefully clean the root system as much as possible, if necessary all the way back to the stem base. Severely prune back the stems so that they are not over-taxing new roots that hopefully will form. After a few days, repot the plant in fresh potting mix. Water sparingly and just in the area of the surviving roots. If rehabilitation is successful, the remaining stems will eventually plump up and new stem growth will begin. Save numerous cuttings of various sizes and try to root these to produce new plants (see Propagation above). The success rate may be very low when rooting cuttings of very stressed plants.

I hope that you find this helpful as you rejuvenate your plant.