What Causes Swollen And Painful Lump On Inside Of Shin?

Hi I noticed a red raised, rounded and painful lump on the inside of my shin a week ago. I haven t injured myself and to my knowledge I haven t bren bitten. Over the course of the week the lump isn t as prominently raised but the swelling has doubled in size and is still very tender. My ankle swelled up yesterday but the swelling there has reduced already. I saw my Dr yesterday who thinks it might be a bite (although I can t find any evidence of punctured skin in the area) and has prescribed antibiotics. It still feels no better after a week could it be anything other than a bite?