What Causes Persistent Cramping Even After Menstruation?

hi, i finished my period the week before (27th-31st) and the next week after started having cramps wit out bleeding my cycle is normally 28 days and isn't due until sometime next week (week of the 18th) i took a pregnancy test today and the test was negative, i want to know if i took it to early or if theres anything else it could because obviously i am still cramping. thank you