Pain at back of knee

I have beeb running "seriously" for about 6 months and recently have been getting a problem at the back of my left knee.  Occassionally when I run it feels like the joint is n't working correctly and afterwards it is sore, this has n't affected my speed but it seems to be getting worse and more regular. When I strecthc my calf out there is a lump at the top of the calf compared to the right knee.  I've tried resting for a week but the problem soon returns. Any ideas? 
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