I did something like that a couple years ago. It took 2-3 months to get to the point that I could kneel with comfort again. Probably would have been fine after that, except I kept bashing it every few months, because I'm clumsy. Doctor says it's bursitis, but not a big deal.
That's probably what happened to you, the bursa, that is, the fluid sac in front of your kneecap swelled up from the trauma.
I've found that heat followed by ice is great for that stubborn swelling. If it doesn't go down, a doctor can draw out fluids with a needle or administer a cortisone shot.