What Causes A Delayed Period With Negative Pregnancy Test?

hi my name is amy i'm 24 and i not come on my period in 9 days and i'm never late i did some pregnacy test and they all come back negative and i got up this morning and i was bleeding so i was guessin my period but i also have really bad pains i never had them before they are so painful and they makin me feel sick and dizzy please could you help