As a kid, I could always taste this in the store-bought milk, as well. I remember nearly every spring, telling mom "the milk tastes like grass!" Mom would explain that this happened when the cows switched from winter feed back to grazing, and by the second or third glass your tastes would aclimate to the change, until the first bottles of the following spring.
I'm not sure if my taste buds have dulled, or if farming practices or milk processing has changed so much, but I no longer notice the change-over.