What Causes Intermittent Feeling Of Knot In Abdomen?

Hi. I m 20. I m 8 days late, I Have a knot feeling in my stomach that comes & goes. My period was suppose to start on the 15th. & its now the 23rd. (I had only ONE irregular period that started on the 17th once but it is ALWAYS on time. Me and my boyfriend have a very active sex life. I took a test on the third day of being late and it came back with no answer , so I took another and it was negative. I also took one today and it was neg. Like I said, Iv be never been late!! When can I take a test again? Please help!