Cat keeps vomiting, please help

Last night my 7yr old female started vomiting in the middle of the night, the first time there was a little food and hair on it but after that she vomited about 1 times more just watery fluid. 4hrs after it started there was some blood on the vomit so I took her to the vet, according to the vet she's healthy, so they just gave her an anti-vomit medicine and order bloodwork and xrays.

The results were ok, bloodworks are normal and xrays are clean. Since then she has been vomiting every 3 to 6hrs, but tonight (it's 5:30am right now) she started vomiting again every 20 minutes. I went back to the emergency room, they gave her some SubQ fluids and more anti-vomit medicine. The vet things it could be a gastritis.

After coming back home she spent about 6hrs with no vomits, then she started doing it again. The bad thing is that my second cat just started vomiting in the same way. So I know that discards many things, but it really worries me.

Indoor cats, no plants at home, Food is the same as usual, cats do not share litterbox, the only symptom is the vomit (now they're not eating but I'm sure it relates to all the vomiting).

Vet wants me to pickup an anti-vomit oral medicine on the morning and try that to see how it goes, but the fact that they don't really know what's going on really freaks me out.

Any information you can give me will be really appreciated, thanks!

I'd get a second opinion. Sounds strange that both cats would now be doing it if it was just gas problems.

Could she/they have gotten into something toxic? I know not plants, but cleaning products perhaps?

The anti-vomit medicine is all well and good but it fails to address the cause(s) of the vomiting in the first place.

Completely agree with Yosemite regarding getting a second opinion. The frequency of the vomiting episodes is rather alarming, as you said.

Cleaning products would be my guess, too. Go through your schedule for the last 3-5 days, walk room to room and try to remember everything you have done or used. Write it all down no matter how insignificant or unlikely it may seem. And just because a cleaner may say safe for use around children and pets doesn't necessarily mean it is.