What Causes Late Periods With A Negative Pregnancy Test?

hi, just wonder if im pregnant, my last sexual contact was 1st wk of june, got my period on june 23, normal 1 wk period, then no period on july, so I took 10 provera tablets by august and menstruate after the 10th pill, but this september, no period again, my tummy is bulging, and my breast grew bigger and heavier, already done 5 pregnancy tests, the last two was just 2 days ago..all results are negative..