My puppy's been throwing up white foamy stuff all day. What's wrong?

She's a 11 week old, Australian Shepherd. She hasn't eaten or drank anything since this morning. She's thrown up about 10 times today, and it's obvious she doesn't feel good. Maybe over-eating, or something serious? I don't know, but she'll be going to the vet tomorrow for her shots so I'll have her checked out then, if she's not better. Thanks!