Vaginal Spots.

Hey, i'm a little unsure about how comman it is but sometimes i get the odd spot which usually bursts the same day it appears on the outside lip of my vagina. This time however i have a rather larger spot further in towards the back of my vagina about an inch from the lip. It has burst twice now with normal spot like yellow puss. Is this just normal or could it be something more serious? I get tested for every std ever 3 months and i've only had sex with 2 people since my last test came back all clear and was protected with both so surely it wont be a std. Just want to check that spots down there are normal.

Thanks

xx