What Causes Difficulty In Getting Erection?

Hi there, I have a few questions about my genetalia and getting an erection. I used to have pains with my testicles a while back on a few occasions but that has now stopped. The concerm I have is that my left testicle has shrunk to a pea but my right is still the same. Another worry is I haven t had an erection on my own in a while now, the only time I can is when I masturbate. I m seriously concerned and wanted to get some expert help before going to see my GP. Many Thanks, Corey