Dear friend, welcome to HCM.
has the lump been there?
Was there any trauma to that bone?
Does it hurt on walking?
Is the pain constant or in bursts?
Is the lump bony hard and movable or fixed?
If immovable, hard ,and dull aching , it could be a bone outgrowth.
Get an ortho opinion and an Xray of Tibia-fibula
AP and lateral views to see what it is.
bleeds or soft tissue
tumours sometimes come like this.
Would like to help you in once you tell me what is the Xray like.
All the best. Take care.