It could be Gypsy hurt herself when she slipped and that's why she howled afterwards. I recommend resting her for the next 48 hours. No running, jumping, or playing. Take her out to toilet on a lead only, and bring her right back in when she's done. If she still appears painful after this time, then I would take her into the vet for an exam. The vet can diagnose where the pain is coming from, and can dictate a treatment plan for it. She may also benefit from some pain medications, which the vet can prescribe. Please do not give her any human pain medications, as many are toxic to dogs and can be fatal if given. I hope this helps and Gypsy feels better soon!