Overdue but not pregnant

Hi everyone.
I'm F. and I'm 25. I stopped the pill in December. Before that, I had nice, regular periods with a very regular cycle. I could have set my clock by it! Since I stopped, things have been a bit wobbly (my 'stop week' shifted a week later) but that settled down quickly again.

Officially, I should have gotten my period 3 weeks ago. I didn't. A day late, there were 4 days, and I took the step and did a test. Negative. Okay, not pregnant then. So I decided to wait until the coming weekend. So be it. The second Sunday arrived. Still no period... weird. After 2 days, I did another test. Negative. Could I have been mistaken after all? I looked into everything with my boyfriend and I wasn't. I should have gotten my period a long time ago (I keep everything neatly tracked on a calendar). Waited again....

Now we've passed the third Sunday and I haven't seen a single sign of my period. The test was negative again.
So we're 3 weeks further and I'm starting to get a bit worried...

I don't want to test again (I already know it's negative) and I've contacted my GP who doesn't seem to take me entirely seriously.
Does anyone have ideas/tips/tricks or anything else for me?

Thanks in advance!
Fujiwara