Late AF

So my AF is now four days late, I’ve had next to no pms symptoms. Last month I was a week late for AF, but I’m on the pill so it’s becoming very frustrating. I took a test this morning and it was negative. Me and my husband want to start TTC around June, but I don’t understand why AF is being so odd. I do work in an all woman environment, and I work all day (8 hours) with one woman in particular and she just started the pill, so I’m wondering if our cycles are trying to sync up because her cycle has been off as well. Any thoughts?