I recently gave birth to first child 12 weeks ago. Last week I began my first period since giving birth. It has been very light. Last night I began cramping extremely bad from my lower stomach into my thighs. My hips felt similar to the pain I had when in labor. I went to the restroom and passes a large tissue substance. I brought it to the er but it will be a few days before they have test results. They did a pelvic exam and said everything looked like I was having a normal period. And my urine said negative for pregnancy. I'm just curious as to what this could have been.