Mixed signals

I'm new to Glow, but hoping for some advice. We've been officially  ttc for 3 months now, I've been off the pill for 4. 
I thought I regulated well after getting off pill because my periods were still coming right about 28 days apart.
This month, my period didn't come. I waited a week, excited that we might be pregnant, but pregnancy test was negative. My period finally came, on day 40?!?! But if only lasted 3 days (normally 5-6).
I'm nervous about this change in cycle length and being able to catch my ovulation window. Any advice on what to do or what may have caused this change to what "normal" had been?