Blip in my cycle - any ideas?

[Marvellous]

I've been off Depo for about 2.5 yrs - it took a few months for my periods to return but they've been totally regular since they did; 30/31 days every single time ... apart from the month before last, when it was 55, then went straight back to 31. I don't think I was unwell or particularly stressed, and the period seemed no heavier/lighter than usual. I had had sex mid-cycle (mid usual cycle) but we used a condom. I am baffled and a bit worried.

Perhaps a cycle where you didn't ovulate? This can make it drag on for ages as your body sort of 'waits' to ovulate and then eventually - if you don't - gives up, so you get a period and it starts again. Apparently most women have an anovulatory cycle every now and again so not uncommon and not a cause for concern unless it keeps happening.