What Causes Reddish Lump On The External Side Of Vagina After Intercourse?

I have a weird pimple-looking thing on the outside of my vaginal area after sex , it looks like a pimple, except it appears to have a red/pinkish look on the inside, rather than having pus. I'm really freaked out, I don't know what it could be, but it hurts really bad when touched, it appears 3-4 day after sex but goes away after a week, this is the second time its appearing.

Thanks in advance for the help.