Would you believe I used to a veterinary nurse when I was 13/14/15?
It could be any number of things, if the cat's lively and not acting strangely then it's nothing to worry about (gently lift a gum and press the gum to see if the colour returns promptly and is nice and pink... this is a good indicator for infection, general health or digestive problems.)
Does the cat get wormed? Eat any wild animals? Any treats? I'd change the diet completely (something for senior cats may be temporarily beneficial as it's less rich and would encourage the stomach to calm down for a bit). Failing that it could be too much grooming on the cats part, too much dairy (cats should have water, no milk/cream), dodgy/cheap pet food or a mild stomach bug.
If that cat starts looking off colour or the sick contains odd elements (blood, excessive bile, only saliva, etc) then I'd take it along to your local vets for a check-up.