I'm guessing, but my guess is that your caliper piston pushed out past the square seal. You might be able to work it back in, but my advice is to pull off the caliper and remove the dust boot, and take a look at it. IF you decide to go that route, buy new boots and square seals (a caliper rebuild kit) because the chances of getting the your used parts clean and back in without aggrevating a failure in the near future are slim.

If you have never rebuilt a caliper, a drill and a cylinder bore cleaner (three stones on spring-loaded arms that fit into your drill) are handy but not absolutely necessary. You can get by with some emery cloth to clean the inside of the caliper bore. Don't use anything abrasive on the shiny surface that the square seal works on. Use only brake cleaner spray and cloth to clean that. The caliper rebuild kit will come with grease that is compatible with brake fluid. That will make it a LOT easier to push in the pistons when you are done.

When you remove the brake line from the caliper, use a rubber plug or an old (clean) golf tee to plug the hole so the brake fluid does not leak out while you are working.

This is not a hard job, just messy. Brake fluid removes paint, stains concrete, and feels really icky on your hands.