I went from a C cup to a H cup. I have to have a custom made bra. Mine started leaking about that time and its completely normal.
I'm in the same boat as you I've had DDD's for years and with both my pregnancies they just got bigger. Its very normal for boobs of all sizes to leak though, with my son I started leaking around 20 weeks it lasted until I was about 38 weeks then started again the day before I went into labor (39w4d) I'm now 20 weeks with my second and I started leaking around 15 weeks I've stopped for now but still put nursing pads in just incase. You can get cloth nursing pads that can go through the wash so you don't have to buy a ton of disposable, if you plan to breastfeed it'll save you some money. :)
It's normal mine started leaking with this baby at 17 weeks.
It's normal... Kinda a pain that it happens so early but it could stop before you have the baby, start again, be lighter or heavier!! My breasts leaked about 24 wks in but now that I'm 37, nothing! I wouldn't worry about it at all just another 'joy' of your changing body. Good luck