What's wrong with me?

I had a m/c in February 2016. We've been unsuccessful with trying to concieve for 3 1/2 months. I've had 3 cycles since my M/C. I don't understand why its so hard to get pregnant. I have a 7 month old son and it was a miracle that we had him. But now we want to give him a sibling and its just not working. Should me and my husband keep trying or go to my OB and see if there's something wrong with me? I'm still hopeful since I haven't gotten my period this month yet. But I've taken FR tests and they came out negative.