Are you sure you're running on empty?
At 12 weeks, your DD will be having a growth spurt and will be feeding more than normal to fuel this and to indicate to your body that she wants more. It might take them a few days to catch up with her.
Also at 12 weeks, your body is also getting used to the demands your baby is making of it and so you won't be getting as engorged as you did in the first few weeks as your body begins to make just the right amount for each feed.
Demand feeding will ensure that you make enough milk as suckling encourages the breasts to make more. Remember, boobs are never empty, they're constantly making milk so while your DD sucks, she will always be getting something. If the flow is too slow and she's getting bored, try switch feeding. When she gets bored and comes off one boob, swap her straight to the other, when she's bored of that one, go back to the first boob etc. I could go between 5 or 6 swaps when DD was that small (she's 6 months now) to keep her interested. It works by giving each boob a little time to fill and increasing the flow of milk for those first few sucks.
Make sure you're eating well and drinking plenty. I know Tiktok (MN BF Guru) says water makes no difference but I've always felt like it helped me if I'd drunk loads.
If you're still worried, you could try getting hold of some Fenugreek tablets too which will help improve your supply. Be careful with them though, they made my DD a little sick when I took them when she was tiny. She was OK when I tried them again last week though.