I Knocked My Shin Below Knee Three Weeks Ago ,

I knocked my shin below knee three weeks ago , large swelling and bruising , it was sore but not too bad , little pain for first couple of days , the 2nd week no pain just bruising , however at the end of week 3 the hard lump on my shin has really started hurting all the time , it is only the hardlump on the shin that hurts , why has it just started hurting after three weeks ?