Well, from a v quick search it would seem that it is fine..........one site says
Red raspberry assists in the plentiful production of breast milk. The high mineral content of raspberry leaf assists in milk production, but...
...Its astringency may counter that for some women, so use caution
Maybe that's because LO wouldn't like the taste of your milk if you drank it?
Another says.........
Breastfeeding mothers should stick with herbal teas that are not full of potent pharmacologic preparations. Orange cinnamon, lemon lift, and raspberry teas are good choices. Another fine choice is tea made from rose hips (the berry-like formation left after the rose pedals drop away), which contains a lot of Vitamin C.
It's not 100% clear (prob coz there isn't definitive research!...........Maybe another question for MW i guess!!
Most info is about it's use before the birth