I got a couple of surface cavities a couple of years back (the type on front of the tooth, up near the gum line). It was caused by drinking protein shakes when I would wake up in the middle of the night. But it sure wasn't the protein. I remember after seeing the dentist, I looked at the bottle, and sure enough there was a crap load of sugar in there. I switched to regular plain whey for my mid night shake and never had a problem again. I'd guess your shake has sugar in it as well. Switch it for a better quality one and see a dentist more often!! That's way too long between visits bro.