What Causes Hard, Painless And Itchy Bumps All Over My Body?

Okay so, all the sudden I started breaking out on my body. It looks like mosquito bites, but they're in groups. They're hard, painless, and itchy. I have some on my upper stomach under my breasts, on my breast, and under my arms. I have none on my genital area, but I'm scared this is an STD. My mother told me it could be hives because I'm stressed and I have my prom tomorrow, but I'm really scared and wanting some advice as to what this could be..