What Causes Delayed Periods With Negative Pregnancy Test?

i had been on the pill for 2 years and last year i came off due to it not agreeing with me and it was 14 weeks before i had a period i had tests and nothing come back, i then went back on the pill for a month as it was a different pill and come off again and i have now not have a period for 12 weeks i have done a pregnancy test and it is negative anyone know what this could be????