Could be any number of things, worms definitely being one of them! If your cat had been seen by the vet in the last 6 months then you should be ok to buy worming medication over the counter at the vets. Try this first if you do not routinely worm them. You really should worm a cat every 3 months.
A neighbour may be feeding her, she may have ingested more fur due to having a thicker winter coat, there may be a more sinister reason but worm first then wait and see for a few days, check her poo for the dead worms, if she continues to vomit then vet. But hold off if she is not displaying other symptoms, as fees can quickly escalate x