Could this also mean, "You have been drinking cold milk?" I thought रहे हो could also mean "have been"...
Yes, it can mean that as well.
Technically, that would be तुम ठंडा दूध पीते रहे हो though तुम ठंडा दूध पी रहे हो is sometimes used instead.
No it's present tense.
Is it rehae because it's not a molecule of milk but many, instead of reha even though there is only one person drinking the milk??
No. तुम has to be conjugated like a plural which is why it's रहे.
My answer is exactly correct, but is marked wrong. I am confused.