Hi there and thanks for your question.
If your vet has advised no food till the morning, it is best to follow their directions.
Often when there is diarrhea, if we have a period of fasting it will give the intestines time to heal and will make the diarrhea get better sooner.
I would not advise going against your vet's directions! Glad to hear that your cat is feeling better though!
I hope this information helps! If you have more questions regarding this problem, just hit reply. If I am online I will respond right away. Otherwise I will respond as soon as I come online again. Or, if my answer is sufficient, then please hit accept. Thanks!
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would highly advise contacting your regular veterinarian.