Left Knee Pain (Runner's Knee)

Hi all!
I have been running for over 3 years, averaging about 23 miles per week on a 7:35 min/mile pace. I did a marathon on my own back in February and a week after I had a terrible pain in the left lower side of my knee. After resting a few weeks I did some minimal training (maybe 65 miles in total with one 20 miles run beginning of April) to keep me a bit in shape for the Boston marathon last Sunday. During these runs I never had any pain at all so I thought I was healed... :)  
The marathon went wel but on mile 10 I had terrible pain on my knee again, I took some extra energy gels and the rain also helped to lower the pain. I was able to finish with minimal pain but it still got me worried and now I feel my knee still hurting. Do you have any advice on what could cause this or how I could potentially solve this so I am better prepared to keep running, something I am very passionate about.
Thanks a lot!
Michiel