The puss may have came because of a loose joint or stretched ligament from hitting your elbow at a wrong angle or over extending it. The swelling, i agree, is probably because of a bruised bone. I would see a doctor to get it checked out and be careful of hitting it again until it is healed
You will get sick if it is in there long enough, so, you really should see a doctor and have it lanced so it drains.
Well I thought about that also. Thinking that the puss or whatever is in my elbow is getting old and rotten but nothing has happened to me. Wouldn't I get sick like an infection or some kind of poisoning.
You probably bruised the bone a little bit and when you banged it onto the carseat it probably agravated it. Try some ice for the swelling. But the puss worries me. You should go to the doctor to get that checked out.
I think I would have it checked, you could have bruised the bone the first time you hit it, and now, the second hit, you maybe could have caused a small, hairline fracture.