Tigwig. Years ago I had something similar during a period. It was quite a large piece of tissue and I hadn't had a recent pregnancy and neither was I pregnant. I did have cramps with this particular period which was quite unusual for me. I took the tissue to my GP who sent it off for analysis and it was just normal womb lining. Sqad is probably right, but this is just to say that these things can happen and don't panic. It never happened again.
If it was me I would have a talk with my own GP or health visitor.