Boob Blister?? *TMI* Pics Included

Okay so quick rundown. I am 19, never had a blister on my nipple before. It's perfectly round, SO said it almost looks like a second nipple lol. I've never been pregnant, don't think I currently am either. I had a big day of walking around in the sun yesterday so maybe that was it??

But my question is, have any of y'all had something like this before??

It's not solid, if I put pressure on it it'll squish like if it's an air bubble. I don't want to worry about it but I kinda am. It doesn't hurt, either. I can't find anything online that could help me and I'll be going to the Dr if it doesn't go away 😬