The first thing here is that this swelling is unlikely to be due to a blood clot, the reason being that blood clots are usually deeper inside the leg and would not cause superficial swelling over the knee or the leg for that matter. Clots which occur in teh superficial veins would also cause red lines on your leg because of inflammation of the superficial veins. Secondly they would also cause swelling of the leg below the level of the clot alongwith change in temperature in the leg as well. So this is unlikely to be a clot. Pain below the knee can be referred pain from another cause which ill explain below.
In your case i think it is possible that you may have developed mild bursitis in the region which is actually inflammation of teh bursa which are fluid filled sacs inside the knee joint and in some cases can protude outwards . This would cause swelling as well as pain in the region. This would be effectively treated with OTC antiinflammatory medications like motrin or aleve. In addition apply topical OTC voltaren gel as well. Apply ice and see if that helps which i think it will.
Another cause of this could be local inflammation of the muscles in the region which can cause this problem. This would be treated with the same method i described above.
In short i think this is NOT a blood clot and you can wait until monday to have this evaluated. If there is actual swelling below the knee joint which is worsening or if there is any increase in temperature (marked increase in comparison to the other leg) or tenderness in the calf muscles ONLY then i would recommend the ER. Secondly since you are on diclofenac already, then you should stick with OTC voltaren gel as well as Ice. In addition apply a compression bandage in the region which would be helpful.
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