I would definitely go see a doctor and have a blood test done... they are way more accurate than these home tests you can take :D
Did they do a pregnancy blood test.A hpt should be accurate by now hun,so i would say your not pregnant,but there is most definately somthing going on,your body may not be ovulateing which will stop periods.maybe you have pcos or somthing.this isnt bad cureable.I would suggest seeing another doctor,and asking for blood test.and demanding more thorough tests no wonder your driving yourself mad.best of luck.