I had a lump the size of a golf ball develop under my incision several weeks later. It turned out to be a seroma, a sac full of fluid like a giant cyst. I had to have an ultrasound to confirm and they wanted to make sure it was not a hematoma, i.e., blood instead of just fluid like in a cyst. It was there for a long time. My PCP said she could refer me to a local surgeon (I am in California and my donation was in Chicago) for consultation if I wanted to have it drained or removed. I decided to just live with it. I don't feel it anymore.
I highly recommend reporting this to your nurse coordinator right away and having your attending physician order an ultrasound. You want to be sure it's not a hernia, and that whatever it is doesn't need any treatment. Yours is pretty big.
Congrats on your donation and welcome to the forum! Keep us posted.
Edited to add, we all had odd swelling at that site post-op and not everyone had a seroma like I had. The swelling lasted quite a while and protruded. That was different than the lump -- your might just be the post-op swelling. Better to get it checked out, though!