@delphy Do your neighborhoods have the Rock Your Body NAP active? I've noticed my sims showering over and over and it seems to be a bug with the OCD repetitive behavior caused by NAPS right now. They need to seriously tone it down.
You can try turning off your mods, exit the game, create a new save and see if it continues to happen. If it still happens with mods deactivated, then it's safe to say it's probably a game bug. Right now, a lot of repetitive behaviors like making white cake, stealing, push ups, showering, playing with clay and game cubes seem to all be related to the NAP system.