Trying to have a baby

I’m 21F and my husband and I are trying to have our second baby. I’ve never gotten a regular period, I’ve had it every other month, I’ve missed my period for 2 months before but now it’s bothering me because we’re trying and I’m now going on month 2 without a period. I have gone to the doctors they are sending me for an ultrasound in two weeks/ blood work, I’m having cramps in my stomach like it feels like I’m getting my period and it doesn’t come. I was supposed to get it 2 weeks ago I’ve taken a million tests but nothing. I have already had a baby she is 3. We have only been trying for 4 months going on 5 and they’re sending me and my husband to a fertility clinic in September if we do not conceive… any ideas of what’s going on