I've had this several times after I got Cottage Living- my sim would be jogging or walking with a dog and halfway through randomly changed into swim wear during freezing weather! ☹️
@IrisuChan - Haven't had that exact thing, but in this case it happened every time she was instructed to cross that stream. Looks like that section of water mistakenly got some of the coding that the deeper swimmable sections got. Can't swim in it, but apparently the Sim "thinks" they have to swim across to get to the other side, so they change into swimwear.
