There seems to be plague of hay mites around this year. They are the least harmful of the guinea pig skin parasites as they stick their eggs to the hair especially on the bum end (unlike mange mites, which bury theirs in the skin).
However, the current variety seems to be a bit more persistent to remove than the usual seasonal fur or hay mites, which also called static lice by some, mistaking the egg cases for the actual parasite.
Please see a vet for diagnosis and discuss with him a two-pronged attack with both ivermectin (works better with parasites that actually live in the skin and not just from skin detritus) and some kind of insectical bath.
Be aware that any skin treatment needs two days (48 hours) to get to work before you can apply any other treatment.
Guinea Lynx :: Fur Mites