Do you allow him to go out?if you allowed him he would have eaten something so he has not been feeling well all the best let this be right 

 You should not take vomiting lightly. The occasional vomit could be because of stress for example too much activity after taking meal, or simply a virus/flue. However, chronic vomiting like you describe that your cat vomits ever 2^nd or 3^rd day is NOT normal. It might be a symptom of kidney problems, parasites, or even your cat has eaten toxic plants and/or substances. In this case take her to vet as soon as possible.

 My cat has now managed to go for a week, on average without being sick for the last 2 weeks. He has only vomited when he was outside once and ate some grass and I also recently discovered that someone in my house had been sneaking him treats, which he's not supposed to have. I'm starting to think that when something foreign is introduced into his diet, his system can't take it. Also being an older cat, his recovery time isn't as quick as it was a few years ago.