Lump On Lower Leg After Having Surgery For Broken Fibula. Suggestions?

Got a lump on back of lower leg just below snd behind the knee at the top of fibular. I recently broke my left fibula by a hard blow just 3 inches above my ankle, square to the outside of the leg. Didn t break all the way through, but it splintered it pretty bad, hence the plate, which is about 7 inches long starting at the ankle and up with 9 screws. Surgery to install a plate was done 6 weeks ago. Healed very well, and it felt like it did. Under Dr. supervision I have been allowed to go to street shoes, with a little care to avoid falling or twisting the ankle. Today, I noticed a large painless lump at the top of the lower leg just below the back of the knee and to the outside. Didn t notice it before, although it could have been there. Obviously the lower broken leg has had my total attention. Pretty obvious when compared to the healthy leg. As I am sitting down and talking to you, it had actually gone down to almost gone. What do you think it is? if it hurt I would call it a charlie horse but it is painless. Seems like a big muscle knot when it is at its worst. Also occurred to me, I was wearing the big black boot before and within two days. This lump is just above the top of that boot.