Need help!!!!!!!!!! Extreme contraction like pains at day 30

I'm two days late for my period. Test says negative. I'm at day 30 of my cycle... No period and negative test.
This morning I had extreme 'contraction like' pain for 20 mins. It was very painful like back contractions when your giving birth!!! I've had two kids so I'm sure it felt like that. 
What the heck is up!???? Someone please help.