What Can Cause No Periods In Past 8 Months?

Hi, may I answer your health queries right now ? Please type your query here...hi i have not had a period for 8 months, i hae taken 2 pregnancy tests & are negative.i waslying on my back all nyt & i woke up this morning with a sort of ache in my upper stomach & upper back, i have no pregnancy symptoms. i have feltsome abdominal bloating but didnt know if it was because i havent had a period? what could this be?