What Causes Bump On Outer Lip Of Vagina?

hi, i have a question. I was sick with cold over the past month. Then one day out of nowhere I find a small bump on the out lip of my vagina. It is not painful but it's kind of firm. It does not look like a pimple. It could be accumulation of blood or something. I'm not sure what could cause as i never had any STDs. I also must say that I had vein injections done on my legs a couple of weeks ago. I'm 32 y.o. and I had children.