I Have Already Been Diagnosed With Osteoarthritis In My Left

I have already been diagnosed with osteoarthritis in my left knee and plan replacement surgery next year. For months I have been a "2" on pain jumping to a "6" after walking or exercise. My doctor gave me a cortisone injection 2 weeks ago, and my pain has escalated to a steady 7-8 and 9 with exercise. Suddenly almost overnight I have a large lump on the underside of the knee. Is this lump something I should worry about? I am out of town but on return have an MRI scheduled in a week. Should I be so concerned that I check it immediately?