Even tho i wore underwear, can i catch anything from trousers that had alot of sweat and vaginal discharge from someone else in them. ?

Remote: If your own pants and underwear stayed dry during whatever the course of action was, the chances are much lower that you could have caught some kind of STD or other infection. If your own underwear became wet as well I suppose the chance may be higher but still rather remote. Best of luck.

Not likely: Based on what you describe, I would say you're probably ok.