What Causes Tenderness In The Abdomen And Back Pain After Menstruation?

Last date of intercourse was 6/28/11, last period was short and 7/13-7/14, cramping, back pain and my abdomen has been and still is tender to the touch since my period, especially my right side. I have taked 9 preg tests, the last was 4 days ago, all say negative, today I had blood tinged cervical mucus, could I still be pregnant regardless of neg tests? I have 2 children, both tested positive early, my youngest was pos day before missed period.