Confused

Ok so about a week ago I had some sever cramping and pain in my lower tummy area. I am not pregnant according to the multiple tests I've taken but after the cramping I started what I thought was my period. It lasted no more than 5 hours... My cycles are usually 28 days Andy period lasts for 7 days but that did r happen this time. Anyone have any idea as to what's going on with me?