Sad and not pregnant yet

I have been married 3 years now & I had a fear all along that when i would be ready to begin ttc I wouldn't be able to and sure enough we're not. I got off of birth control as soon as I got married for this very same reason and we relied on the rhythm method. My periods were predictable every month on the dot for the last 3 years. But it seems that as soon as we decided to ttc my periods became irregular. After the 1st month of trying I then somehow skipped the next month and did not get a period, then i got it the next two months but again skipped this month. This month was especially hard because I really thought we were pregnant. I had an upset stomach and no signs of my period but much to my disappointment i wasn't pregnant. So now I have to wait until my period comes in late August to start all over the tracking system of when I am suppose to ovulate. I am trying not to stress and I don't think I am but I am sad. I cry sometimes because I feel it's so unfair but all I could do is hope for the best. Hopefully my menstrual cycle will begin to cooperate. My doctor says that stress can cause me to skip my periods they don't seem to think anything is wrong. But I'm suppose to go back in 3 months to do a full workup.