No period goin on 3 months

I havnt had a period going on 3 months and I have taken several tests but all keep saying negative... I had a few look like they had faint lines but then bought one with plus sign and it was negative... I have an appointment in a week but it's aggravating!!! I'm gonna buy 3 more tomorrow... Could something make them negative? I'm on no bc... I can't think of any other reason why Id miss my period for 3 months when I just got back regular... Tia... :)