Why would I become irregular all of a sudden?

I’ve been pretty regular all my life. My length of cycle is either 30-31 days. After I stopped breastfeeding my daughter my menses returned and I started birth control in 2013. I’ve been off birth control since March 2018. I had an extra period that month but right after I became regular. Every 30-31 days without a fail. My husband and I have been trying to conceive since i got off birth control and have not had any luck. In December my AF was 8 days late and now this month I am 5 days late (pregnancy tests are all negative) this is very odd for me. I’m not use to being irregular. Nothing has changed dramatically in my life. I just turned 30 and I have two kids. The first time I got pregnant was thru the withdrawal method and it only took one time and I was with-child. The second time I was 23 and it took me about two months to conceive. I am going on 10 months of ttc without any luck and now my periods are becoming irregular. Has anybody out there gone thru the same situation? What was your outcome?