My Mol does this quite often and it's always a hair ball. Sometimes she's sick four times before she finally gets it up. Just before she's sick she starts licking her lips repeatedly (at that point she gets pushed off the bed/ sofa/ keyboard) then her stomach starts rumbling, then the retching starts. When she's brought up all her food the sick goes white and foamy. I called the vets a couple of times when it first happened and he said if it happened more than three or four times and went on for more than a day i should call him.
But there again, if it'll put your mind at rest call. When Mol is sick I give her fresh water and hold back wet food for two hours (from experience if I give it to her straight away it just comes back!) if she really winges I let her have a few dry biscuits.
HTH