Thank you everybody. The vet said she has a kind of infection (I don't remember the name, sorry), and that she probably caused it herself by licking too much (my silly girl...)
He gave her and injection with some antibiotic, and we'll have to put some ointment on her paw for a week.
She's been so quiet today (the vet said it would be normal, because of the antibiotic) but I hate seeing her like that. I just want my active, happy pup back!