it's called a post operative haematoma or post surgical haematoma.
some ladies have had this and reported on this forum - use the search box on the green bar to look for previous discussions
haematoma is the english spelling and hematoma is the American spelling for the same thing.
There are some suggestions for caring for a haematoma on the following web page
This is a different issue to endo cells that get stuck in a surgical wound site and start bleeding in line with the monthly cycle, as the endo elsewhere does. Only the blood is trapped in the scar and it does require surgery to remove that endo.
this is called implantation endometriosis, and there have been several cases of that on the forum too.
The hard lumps that they form build up over time and wouldn't account for your lump so soon after your op.
Basically it is a bruise - an almighty one at that. And like most whopper bruises it will normally reabsorb back in to the body over a few weeks and change through the rainbow of colours as it does so.
It isn't the same as a blood clot, ( it is a clot of blood,) but a blood clot is a different thing, that occurs inside the blood vessels and prevents blood flow and is a medical emergency that requires immediate administration of blood thinners. It is what we call a DVT or deep vein thrombosis.
We have had a few of this on the forum too, but they don't normally occur where you had surgery but in your legs or elsewhere remote from the actual surgical sites.