Help! TMI picture

 I started my period last night. I've always had awful cramps, but early this morning it was excruciating. I couldn't move without feeling like I would throw up. When I went to the bathroom this afternoon, I saw what I believe is uteran tissue. I just went to the bathroom again and when I wiped, I got the latter. It looks like there's a bit of clear/whiteish mucus. pregnancy tests prior to my period were all negative. I'm obviously very concerned and fearing the worst. Does it look more like a blood clot, or tissue?