Periods can be all out of sync after having a child. I wouldn't jump to any conclusions. This is normal and not necessarily pregnant or having medical problems.
I was several days late with my second pregnancy before a positive urine test. I'm now 25 weeks. False negatives happen. Especially depending on the time of day you take the test and how much water you've had. First morning urine has the highest pregnancy hormones and usually works best.
It's worth checking in with your OB-Gyn. They can run a blood test and check and ultrasound for final confirmation of pregnancy or not. They can also do more extensive hormone testing. Sometimes unbalanced hormones can affect your cycle.
Good luck and congratulations on your daughter.
I took numerous hpts after being 4 weeks late and they were all negative. I even did a serum pregnancy and it was negative. Turned out I should have waited one mode week to test. I had my first ob apt today and I am 8 weeks pregnant. Everything looked good on the ultrasound. I would wait another week and try to take another home pregnancy test. Could also be your hormones out of wack and needs to be regulated by a birth control pill. Good luck to you