A hard immovable lump on my lower leg.

I've had a hard immovable lump on my left leg, 3 inches right above my ankle for more than 10 years. I've also noticed my left calf is a bit bigger than my right one (never worried me as it has been that way for years). Apart from painful to touch, I had no other problems with this lump until recently. It's sometimes painful though not to the extent I can't move my leg. It is just a little pain scale 2 of 10, I'd say. But it happens occasionally and the lump has gotten bigger than before. I can see it protruding a cm on my skin. My sister told me it was not that visible before. I remember first noticing it as a flat bruise, but it won't disappear after rubbing it. I got inflammation after rubbing it for days and stopped doing it. The other time when I saw it again, I found this lump. It ever hurt to the extent I couldn't even stand when I accidentally had it knocked very hard. I'd be really thankful if you could help me with some information on what this lump can be. Thank you