Plum tree infestation
Leaves on trees curl if heavily infested with aphids. Ladybugs and their larvae feed heavily on aphids, so that's another clue. I'm guessing you have aphids. Look at some leaves closely. It helps to uncurl a few over a piece of paper, and have a hand lens or magnifying glass to look closely at the leaf undersides.
Usually there are live aphids, cast skins, and possibly some aphids with wings. Look for ladybug larvae also (internet search so you recognize them).
Let us know what you find. It is too late to heal these leaves, and possibly treatment will need to wait for early next spring. I'm not sure yet.
Is this a tree you could treat, or would you hire a tree service if spraying is required?
Here are links to a couple of pages from the Pacific Northwest Pest Management Handbook from the Extension Service. It includes life cycle and home management, cultural options.
Plum, flowering (Prunus)-Aphid: https://pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/plum-flowering-prunus-aphid
Landscape pests-Aphid: https://pnwhandbooks.org/insect/hort/landscape/common/landscape-aphid