I had a similar situation and ended up having am ectopic pregnancy..fetus heartbeat stopped at 7 weeks but i had been pregnant with the ectopic for 4 months! I was lucky my tube didnt rupture.I suggest getting a blood test and if they dont find anything, maybe get the pituitary looked at.
I would take pregnancy yests of different brands to compare. A false negative ispossible but not extremely common. The stress and fear of pregnancy could also be affecting your body and irregulating your cycle. You could also look into "phantom pregnancies" which are false pregnancies where the woman tricksher body into believing shes pregnant and starts experiencing pregnancy symptoms. If you're obsessing over thr possibility of pregnancy, this could thecase.
If the test is negative then no. Missing a period for a long amount of time doesn't always mean pregnancy my friend hasn't gotten hers in over 6 months and she's not prego