PP periods? TTC

We are TTC 5 months PP, we had a long hard time getting pregnant with our miracle baby. I've already had my first period and am currently 18DPO, pre pregnancy I had a 28/29 day cycle. I'm on day 32 and have BFN... Cervix is medium/high/soft and wet.

So what's up? Do cycles change that much? And I thought menstruation starts 15-16 days post ovulation. I had a positive opk test 18 days ago followed by a negative opk the next day.

HELP