Anyone underweight and having problems getting pregnant?

I've always been a really small person. It runs in my family.  My husband and I have been ttc for 6 months now.  This is my first month using opks and I never got a positive. My cycles have been between 34-42 days. Today is day 26 and still no positive. I'm starting to wonder if I haven't been ovulating this whole time. Anyone underweight and having issues?  My bmi is about 17.7 and I've read that anything under a 19 isn't good. I'm 5' 4 and 103lbs. The highest I've ever been able to get myself is 108 and that was with weight lifting and protein shakes.