12 day old pups bottle fed acting weird and listless

Lexi gave birth to 8 pups. It seems her milk just isn't enough but how do I know for sure?
I decided to supplement a couple of pups that I felt wern't getting enough to eat.
After I bottle fed 3 of them they haven't been the same since.
It's really freaking me out because I don't know why they went from being wiggly to listless.
I bottle fed them 3 days ago and they are still lethargic.
One pup has vomited twice when I tried bottle feeding her again.
I really hope Lexi doesn't have milk fever because the whole bottle feeding thing is kinda scary and so far it has brought on negitive results.
Lexi sees the vet day after tomorrow.
Also pups have the runs now but could that be from the antibiotics Lexi is taking?

If you are worried about the pups you should see a vet. In pups that young things can get bad very quickly.

A vet is the best place to be. The mothers milk is not full on as yet and won't be for a few days, so you may be giving them the wrong mix, did you take advice, read books, have a mentor, before deciding on a litter. Is there anyone with experience you can ask. Personally I would have been at the vets a few days ago, they could have picked up an infection

What are you feeding them ?

Was she wormed appropriately during the pregnancy?

I wondered that too, as this litter is from a dog (mixed breed) she found abandoned outside her property 3 yrs ago.

I'm thinking Fading Puppy Syndrome here. Lost an entire litter to this many many years, have never forgotten it, has stayed with me, hence my girls ALWAYS having the Canine Herpes Vax, prior to mating and another one 2 weeks before the birth.