Question 🤔

I was supposed to get my period January 29th and I still haven't gotten it. No cramps, no symptoms of it about to show up. I've taken 3 pregnancy tests so far and they've all come out negative. I have a bunch of different apps to track my cycle because before I had kids it was beyond irregular. Since my son was born (August 9th) it has been on the same day every month since I got period back in September. I have one app that says my period is supposed to come tomorrow. If it doesn't happen and this last pregnancy test says negative tomorrow morning then I'm making a doctors appointment to see what's up. Just in case. I was just wondering if anyone knows what could cause a period to be this late? Or, if by some chance I am pregnant, what could cause a test to say "negative" when it's actually "positive" other than low HCG levels?

Thanks in advanced! 😊