If it is not happening often (maybe once a week or so, -ish) I would not particularly worry about it, cats do that from time to time. Or if a meal has been delayed.
If it is happening more than a couple times a week, though, or if you are particularly wanting to stay on top of any possible problems that may develop, you could take her in to the vet for an exam and a blood panel. Kidney or liver or thyroid problems (among a other things) can have recurrent between- or before-meals vomiting as a symptom. From what you say though it does not sound frequent enough that it would concern me much.
Good luck, bless you for giving a FeLV-positive cat a good home,
Pat