Sick Kitty

My 6.5 month old kitten has spit up three times in the past 2 days. Completely clear other than the bit of froth, all within feet of her water bowl. She also started having diarrhea today. She was spayed last Tuesday. I contacted my vet today and took them a fecal sample, which was negative. They gave us a probiotic and said to let them know if she is not better in 5 days. She is acting fine, not lethargic or anything, but does not seem super interested in her food (which has the probiotic in it). I just want my kitty to be okay. Any thoughts?