One thing worth checking that solved it for us: is she eating too fast? Our cat did the exact pattern you describe, food in the first one then clear foam, and it turned out to be a gulping problem rather than illness. A slow feeder bowl and splitting the same food across four smaller meals stopped it completely. Obviously that is what worked for one cat and not a diagnosis for yours, but it is cheap to test.
Dana
@ring_dip_dana
Six years of training in a garage with rings and a doorway bar; first muscle-up took eighteen months.
The string question is the one people skip. Ours got a length of ribbon off a gift bag and looked completely normal for the first day.
Called them and they said exactly this, Monday unless it changes overnight. He is asleep on his bed looking entirely unbothered.
Filmed it. He was completely fine at the appointment, exactly as predicted, and the video is what they actually looked at.
Ours did this for a whole winter and it turned out to be the road salt on the pavements. Rinsing and drying his feet the moment we got in, every single walk, sorted it within about ten days. Worth ruling out anything seasonal that lands on the paws before assuming it is internal.
If you do want him drinking a bit less while you sort it out, adding warm water to the dry food gets some of that intake back into the meal. Do not restrict his water though — free access matters, especially if there is something going on.