Help pleaseee

So yesterday I went swimming with my boyfriend and his family in a pretty popular lake in our state (that you are able to swim in) anyways we were in there for hours they were all jumping in off the ledge and I did a few times but spent a majority of my time just floating around, anywaysss as we go to leave my I was putting my shirt on and I noticed this bruise like thing mind you it never looked like this yesterday it looked like more of a graze and had red dots but didn’t feel like a graze it felt like a bruise (which it is) but the thing is I don’t clearly remember doing anything to cause it it just appeared, I’m pretty sure if I knocked my arm I would’ve felt it but can anyone help me know how it could’ve appeared or anything like that?? Thanks