If youre positive that nothing has changed with respect to your Dose and Tamp, and the beans are fresh then I guess it could be related to your machine in some way. How often do you do a chemical backflush on your VBM mate? Could be that there may be some rancid coffee oils or other matter in the Group above the Shower-screen and Water Distribution Block.
Are the shots still pouring within the guidelines of the so-called "Golden Rule"? Have you changed the type of water you use or if filtered, could the filter require changing? Just shot-gunning really as this sort of issue is usually related to the quality of the beans and/or the way you dose and tamp. Someone else may have some other ideas worth trying. All the best Buschy.... :)