I have not had a period in a week now I'm 25 and my period is never late. My lower back hurts and I have lower cramps also doing sex my stomach hurts could I be pregnant I took two pregnant test the other day and it say I'm not pregnant

It's possible that you are pregnant and the tests just haven't picked it up yet. Or your period might just be off this cycle. Wait another week or so and see if your period still shows. If not, try another pregnancy test. Make sure to follow the directions carefully. If still negative, see what happens when your next period would be due.