Is it possible?

I'm curious if it is possible for a mother cat's milk to make her kittens sick and vomit.

We've been feeding Snuggles' kittens formula for like two or three weeks because she won't feed them more than once or twice a day, her kittens are two days shy of being 6 weeks old.

Well two of them were vomiting so I did some research and at first I thought that maybe they developed an intolerance to the formula because online it says that can happen even if they've had it before so we tried giving them soft cat food, and only three wouldn't eat it and they didn't get sick from it.

But then the next day my mom used bottled water to mix the formula and they all were fine, on the 15th she forgot and they threw up so we thought maybe it was our tap water.

Yesterday they didn't throw up at all after they ate, but while heading to the bathroom I saw them nursing off of their mom and when I came out I noticed vomit on the floor. Then a few minutes ago after nursing off their mom Jose got sick and vomited.

So can a mother cat's milk make her babies sick, is it something she ate or is it because she currently has a cold?

There are a couple of factors that are complicating things here - formula vs. mom's milk; tap water vs. bottled water.

They may have gotten sick from something in the tap water or from mom.  Has anyone seen the vet yet?  They may need antibiotics and probiotics.

We can't really afford to take them to the vet right now, we're trying to get rid of them at the moment.

Tuggles' kittens have also been drinking the formula and then nursing off of their mom and they don't get sick at all.

Snuggles' kittens are perfectly fine, happy and healthy until they nurse from her, then they throw up and don't want to move or play.