You could have caused your underlying arthritic condition to flare up, but you also have acute trauma to the soft tissues around the knee.
If the swelling is outside of the knee joint, then it is from the bleeding and edema from the hematoma (bruise). Deep muscle bruises can take awhile to go away and can be quite painful.
If you actually have swelling inside the joint (an effusion), then you could have injured something inside the joint (or just flared your arthritis). An effusion feels like a water balloon under the kneecap.
You should apply ice to the bruise to slow down any more bleeding. After a few days, if heat is more soothing you can switch. If you can take them, antiinflammatory medicines will help also. Some patients like to apply some compression to the area, to help prevent more bleeding and to help squeeze the edema back into the core. This can be an ace wrap or a neoprene sleeve. Just don't make it too tight and cut off the circulation, causing more swelling below the knee.
You will also need to stretch the quad muscles and the hamstrings, so they do not scar and contract. Be gentle, so you don't make the bruise bleed more. But, it is really important that you do stretch.
Then after the bruise has mostly resolved and it doesn't hurt to stretch anymore, then start strengthening.