Don't panic I've had 4 c-sections, it's the congealed blood lying in the womb, you will pass several for a few weeks. As long as there is no heavy bleeding for several weeks(More than 7 pads a day) Follow the instructions the nurse gave you.
I Had A C-section 3 Weeks Ago, This Morning Passed A 2 Inch Long Blood Clot, What Could This Mean?
Stale blood left after the operation that has clogged it is a good sign,it means you are not bleeding internally just follow Drs. Orders and you should be fine and don't be overly active good luck