I Think My Dog Is Pregnant, She Mated 5 Weeks Ago And Is Gaining Weight. If She Is 5 Weeks Pregnant, Should She Have Milk Already?

No, not that soon, she will start having milk in the later stages of the pregnancy. In the 8^th or the 9^th week I'd say. If your dog is in the fifth week of pregnancy then it is quiet possible that her nipples have started to get enlarged. Apart from that, the fur around the nipples and on the belly will start to get thinner and thinner day by day. She might even be having morning sickness by now due to the hormonal changes encountered in pregnancy. In my view, this is the perfect time for you to take your dog for a physical examination. The vet will tell by palpation whether the dog is pregnant.    

Mammary development can occur as early as 3-4 weeks into gestation.  Lactation (milk production) can begin 2 weeks or earlier before whelping (birthing).
 

I my   dog is 8 week pregnant and she is not putting much weight on will they be under her ribs

If she was having a false pregnancy her vulva would not be swollen and she wouldnt produce milk she would just get a enlarged stomach and act funi hope this helps :)

Her stomach will probably have bumps or the puppies would move inside of her