Please Help Me. I'm So Worried :(?

I'm going to the doctors on the first of July to get tested for an STD because for the past 2 months or so, I've been getting a lot of discharge that hasn't gone away even though I've been using Vagisil. Now, I'm even more worried because I just looked at my vagina and I noticed a small blister/bump located on my left outer labia, up near my clitoris. I had noticed when I shower and I would clean down there, it would sting just in that one spot, but not at any other time...only in the shower. It's quite small, barely noticeable. I looked up pics of herpes and this bump or w.e it is looks NOTHING like those pics. It's not huge, oozing liquid or anything...it just stings when I shower. Could this be herpes? :( The last person I slept with was my ex bf back in March and from what I saw he had no herpes...but I believed he passed on a STD to me. Please help me. I'm freaking out.