Help!

My whole life had regular 32 day cycles

Even when I had IUD I was regular

Had IUD out and got period 2 days later

Went back to 32 day cycles

Had a blood test that showed i ovulated

Then this month opk never showed smiley face and now I’m 4 days late on period... never been late in my life!

Taken two tests and negative...

Haven’t been stressed, the only thing that’s changed is that I’ve flown to the UK for a week... could it be time zone change?!