I'm freaking out please help!

Ok first off I know I'm clean I'm checked every six months when I get my birth control plus I had a baby in January and was tested right before her birth. I started my period today and I noticed a bump down there just on the inside of the opening. I tried looking and it looks like a blister like when you get really bad burned from what I can see and it's maybe the size of the tip of my index finger. It's not painful but now I'm paranoid! I'm a larger girl so it's kinda difficult to see down there. I'm freaking out though and it wasn't there yesterday morning when the husband and I had sex. Could it be from starting my period? Sometimes I get a small little bump (just one maybe half the size of a pea ) on my inner lip down there when I start but it's few and far between and goes away after a day or to. Please help should I see if it goes away before jumping to conclusions?!?

Heather: Wouldn't it still show up on a blood test though? And does that mean my daughter could have it as well if I have something? I've had a pap before when I had the little bump on my inner lip and they weren't concerned and nothing came back but this bump is different?

UPDATE: it looks more like a boil not a blister. Sorry didn't really have a word for it until I was speaking with my husband about it.