My guess would be that you are not using your back as much to draw the lighter weight bow, thus placing more strain and weight on your shoulder. The best example of a way to compensate for higher poundage is a picture I took of Mark Stretton at a longbow event we happened to see at Richmond Castle one day:
You can clearly see the forward balance he uses to help compensate for the very high poundage of the bow he shoots (somewhere in the vicinity of 150#, I believe). It helps to ensure he uses the much larger and capable back muscles, rather than the non-load-bearing muscles in his shoulders.