What Causes Knee Lumps With Bruising, Swelling And Redness?

About a week ago, I had a couple of bumps show up by my knee. And since then I've had some more show up below my knees. They are on both legs and are about the size of a quarter or half dollar. They are red, but don't itch. They feel more like a bruise. In fact, as the redness starts to fade and the swelling goes down, the look like bruises. I have psoriac (spelling?) arthrisis and I'm wondering if it could have anything to do with that. I'm also allergic to sulfa and I'm wondering if maybe it's an allergic reaction. I'm female, 48 years of age and in good shape. What do you think it could be?