Cat throwing up clear and foamy liquid

For the past couple of days my Wallowitz has been throwing up a clear and white foamy liquid several times per day, and was lethargic.  This is very unlike him, he is almost three years old and has had maybe had 1 or 2 hairballs, but other than that he never throws up and is quite an energetic, playful cat.  My husband left with Wally last night to take him to the emergency pet clinic.

We recently got a new houseplant and we put it outside last night after Wally kept throwing up wondering if that might have caused it.

 Poor little guy was poked all night.  They took his blood and gave him an anti-nausea shot. His blood work came back normal, vet said if he keeps throwing up after the shot to take him back to get an x-ray, because it could be an obstruction in his stomach. 

This morning he threw up a couple more times, so my husband took him to the vet.  He had an x-ray done, and the vet didn't seeing anything obstructive.  He said there was gas, but it didn't look abnormal. Wally was given some medicine and a hydration shot, and a bland digestive care food, which he said to just feed a spoonfull a few times a day.  Vet said that if it keeps persisting they can do an ultrasound. My husband did mention to the vet that our other two cats have been sick, but they both had a URI, they both were sneezy with a watery eye with normal appetites.  Ludo had it first, and now he's better, and now Daisy is the sneezy one.  

My husband fed him the spoonful of the food, and a little while after, he threw up again, but it wasn't food. It was the foam again, we thought that was odd.

If he's on medicine for nausea, that makes me think that he's not throwing up just because of an upset tummy. 

I hate seeing my babies sick, has anyone ever dealt with something similar?  Could it still be a hairball, even though the vet didn't see anything obstructing? Does this sound like something normal, or something more serious?  It's very abnormal for Wally to be throwing up at all.

Many of us have dealt with these same symptoms and it is frustrating.  There is always a lot of diagnostics.  My IBD kitty was like this when he would have flares and it took 6 years and several vets to get the diagnosis.  I would get the ultrasound done as soon as possible unless you are one of the lucky ones and it is just some kind of virus and he gets better on his own.  Cats can't go long without getting calories because of the way their liver works.  The liver isn't very efficient at metabolizing their body fat.  The body sends fat to the liver faster then it can metabolize it.  Deposits of fat build up on the liver causing something called Hepatic Liver Disease or Fatty Liver.  My kitty had it and he had a feeding tube for about a month.