Pelvic Pain?

So I've been having some pelvic pain starting in on my left side then going to the right for about 3 months now. I've went to the dr already and they did an ultrasound and said everything came back normal, but they didn't want to do any further testing because they think it was just muscle strain, which I don't think it is. The pain is very mild and dull and I have it when I cough/sneeze and sometimes when I walk. Any idea on what it could be?