M.H.
The pregnancy test may not have been good enough to detct the low levels of hormone at that early stage. It sounds like you are pregnant, and cramping can be normal for some people, as you know all pregnancies can be different. Your two choices would be to either get another test (some of them are good five days before your period is due, therefore detect the lower hormone levels) which may now work, or go to your doctor where they'll do a test for you.
Good luck whatever the outcome.