It's not uncommon to have colostrum during pregnancy, even as early as 25 weeks especially if you've had a baby before.

I'm not sure about the preterm labour bringing your milk on early...it's possible I guess but not really likely. Do you actually have milk there or is it just hard boobs and colostrum? If it's just that I'd guess it's hormone related. The signal for your body to go into full on "milk production" mode is sent to your brain when the placenta separates after the birth.

If you're worried though, you should press your doctor for some input especially given your history of bubbies coming early?