It could be all the above. Make sure to feel the soil before watering - you will want it to dry some before watering more but not get bone dry to the point of wilting plants. Do they have drainage holes? Begonias like that would probably prefer more morning sun and then some relief from hot after sun.

Do they have slow release fertilizer in the potting mix? That could help sustain health as you water and the nutrients leach out of the soil. 

As the leaves and flowers fade you can pinch them off to help new growth come in. 

Let us know if symptoms worsen and feel free to send in more photos. 

Emily