The network hashrate on LTC (so probably other scrypt coins as well) has tripled since 1st dec (was 32Ghs, now nearly 100Ghs) - blame bitmain for releasing so many L3+...!

Diff should make NO difference to rewards - it purely alters how often your miner submits a share so reduces network traffic, the submitted shares are multiplied by the diff to get Diff1 equivalent shares.  teh pool will have a target rate for share submission so will alter your diff to manage that.