The "SHOT" spoken of is Oxytocin and causes (painful) contractions of the uterus; usually given to 'expell' any remaining placental or fetal remains OR to induce labor. The same contractions of the uterus is made by the nursing and suckling of the puppies.
I also don't think 8 hours for a large dog with (was it?) TWELVE puppies is completely out of order. General rule of thumb.. more than an hour between puppies.. get worried, call vet! So, you had 4 hours left to spare!
I would watch that she is eating (yes, give goodies i.e. canned, etc.) nursing, cleaning pups, drinking plenty of water -to make more milk-, and going to the bathroom regularly.
If she appears normal but just a little sluggish for a couple of days, that is to be expected as well as some 'spotting'. And the bigger the dog....??

Childbirth is an overwhelming, but natural process. The dog will probably be fine. You might also ask your vet for some vitamins to suppliment.