Woke up to something uncomfortable

I (30F) went to my boyfriends (30M) cousins house this past weekend. Him and I have been dating for a year and a half, I've been to this house before quite a few times. We were all drinking, and I ended up drinking too much to the point I fell asleep/blacked out. I woke up to my boyfriend angry, I was confused. He starts asking me why my pants and underwear were missing so I look down confused to see I was in fact bottomless on the couch for anyone to see! My shorts and underwear were found in another room away from me. I'm extremely embarrased, confused and hurt that this happened. I'm not even sure HOW it happened which is driving me crazy. He now wants to break up because he thinks I might've done something with one of his cousins. I'm heartbroken and have no idea how to handle this. Again, I also have no idea or recollection on how I ended up with no shorts/underwear, last thing I remember was listening to music with one of his female cousins in that same spot.

Some things to add:

-I plan on going to the doctor to get checked for STD's

- I didn't feel weird or sore down there

Edit: My boyfriend and 2 of his cousins were outside most of the night, which is why he didn't notice. At some point my boyfriend and one of his male cousins went to the store for more drinks, leaving one cousin awake and at home.