I just found out I was born with only one testicle.

I was born premature, so I'm sure there's some complications. I'm 17 in 2 months. I was with my girlfriend in my room, we were about to do the act, and she pointed out that I only had one testicle. I asked her what she meant, and she elaborated. I panicked and ran to the bathroom. Now I'm here, cryin, and I'm wondering... 1) why I never noticed, but most importantly 2) are my balls gonna come back? Sorry if I'm not making sense. I'm in a whole lot of pain, so I'm not really thinking straight.