I Went To The Bathroom Thinking I Started My Menstral

I went to the bathroom thinking I started my menstral cycle with cramps in my stomach. (Which was not my ms) I peed out something the size of a rock (didn't hurt coming out, just a little pressure) it was covered in dark brown, slightly red goo. I didn't look too hard as to want i peed in the toilet as to I was at work... But I'm concerned what it was that I peed out. Any ideas ? Before I call my OB to make an apt.