Is it STD or just a yeast infection?

This is kind of scary for me, and I'm very nervous about bringing this up on the world wide web.. But I want answers right away and this is the best way I can think of at the moment.

I am a 15 year old bisexual female with a girlfriend. Earlier in the school year, me and my girlfriend had sex. A few months ago, we went again. My problem is, that now I'm really itchy down there. It burns just a little bit, especially when I go to try relieve myself of the itching. It occasionally feels like I may have cut something somehow, but that's likely from itching. There is also some white discharge. Anyhow, I told my teacher about it, and she asked if I had a rash. I told her I didn't, and she said it was probably just a yeast infection. however, when I came home early from school today (I couldn't stand the discomfort and got picked up), I went to take a shower. I looked down and there was indeed a rash. Tiny red bumps,each about the size of the tip of a ballpoint pen. They don't hurt at all, though. A couple days ago, I had taken a shower and there was nothing there. The itching had also been subsided for about an hour or so. Then it came back. At this moment, I don't itch, but that's because I just got out of the shower.
I also don't know the full extent of my girlfriend's sex life, nor any details on her previous boyfriends. I have also never had sex with anyone but her.
So now I need to know.. Is this just a yeast infection like my teacher said? Or is it something more?? And if so, is there a way to get rid of it? And more importantly, is there a way to get rid of this without my mom knowing or suspecting I've had sex?

[Mitch82]

It could indeed simply be a yeast infection, the symnptoms you have suggested sound right on par,however it is difficult to ascertain on a website, my advice would be to approach your mother and say you think you have a yeast infection, then get her to take you to the doctor, you can speak confidentially about the issue with your doctor, alternatively you may wish to see the doctor without involving your mum, if this is possible.  Also if you get desperate, local chemists may provide treatment options, as yeast infections are very common, I am not in the US and am unsure of the exact options in your country.