Blood tests

SDPxx
So I went to the doctors about 5 months ago as I have been Ttc for a year now and nothing. My cycle is all over the place I can go months without a period. At this point I was having a period. She tested me on the 1st day of my period and the 21st day they both came back normal except on the 21st say it came back no ovulation and the doctor told me I wasn't ovulating. But what I don't get is how on earth she knows I ovulate on that day if I don't ovulate on that day it's not going to show up is it. I don't understand how she's come to the conclusion that I don't ovulate at all! I'm now testing myself everyday to see if I get any ovulation. Does anyone else understand that?