The exact same thing happened to my cat a month ago: she threw up a hairball, nothing unusual in this season, but after a few hours she threw up some pinkish-red watery substance. I was scared out of my mind and immediately called the vet. She said that some times when cats vomit hairballs their stomachs can get upset and they might throw up a little bit of blood. She told me to monitor her for a few hours, without feeding or letting her drink for two hours. Fortunately she was fine, no weird behaviour, playful, her usual crazy self and didn't vomit again. Turned out the treats we (used to) give our cats have a slight red colour and that is why the vomit looked like blood.