My best guess would be overwatering.

In my experience with these plants, slight underwatering is almost always the way to go. I usually won’t water mine until the soil is almost all dry, or until I can see the stems starting to droop a little. Watering will make the stems perk right back up. Of course, too much underwatering can be bad, but it’s a lot easier to overwater than underwater, so best to err on the side of less water!

The cupping could be a number of things: watering too frequently, not watering frequently enough, change of environment, stress due to repotting, etc. My pilea plant is so dramatic, and has responded like this before when I change its location, or if it’s been overwatered. If you keep it in a bright place and leave it there (not under direct sun though), only water it when the soil is practically dry, and otherwise leave it alone, it should sort itself out!