Membranes can and do get retained sometimes...it's happened to me, a fairly large piece of it, too. It did, however, come out on it's own on day 4, no problems. If I am 'seeing' what you're describing, then it is retained membranes.
Do watch your temp and be alert to pain in the uterus--NOT afterpains, contrax that come mostly with nursing/right after nursing, but a throb, ache, constant. Any fever is suspect--but if you get an infection, you will probably really be hurting, too. Any weird smell--if there is infection starting, that would be because what is in there is rotting, and it will smell bad. Watch your lochia/flow--for yellow or green. Red, brown, clearish, pinkish is ok and it should only smell like blood, not rot. You should not be soaking pads very fast--or at least, you might have a couple hours of more bleeding when it tries to exit, but this should subside--get help if you bleed so much that it makes you feel faint or otherwise icky.
Rest, excellent nutrition and hydration, LOTS of nursing, and if you're up for it, then maybe a little walking (easy)--it will most likely come out on it's own.
Just don't take any symptoms described, or any feeling of being ill, too lightly. Intrauterine infection can kill--pretty darn fast. That said, don't worry either--just pay attention. You may or may not notice when it passes--I did, happened in the shower, if it's a big enough piece it'll be hard to miss. But it could break up while in there, and come out on your pads in such small bits that you miss it. If it's only membrane, most likely it'll pass without ado.
I drank ginger tea to help it exit--didn't exactly know it was in there, but had a feeling I should treat myself as if something was in there that needed to come out.