OUCH! Help!

So the start of the week I decided to try and wear a thong I haven’t worn in agessss due to this happening the last time I wore them. I don’t know what this is but the thong is quite tight on me (perhaps a little small now 😅) and I’m guessing it’s some sort of blister or sore from wearing really tight underwear? Anyone got an idea to what it is? It’s on my outer vagina lip.