Spirit Airlines Inc. flight 1101 came to a sudden stop while taxiing to their gate at Orlando International Airport (ORD), Orlando, Florida, and a flight attendant (FA) sustained a serious injury. The flight crew was instructed to hold short of the ramp area due to another airplane occupying the alleyway to their assigned gate. Once the other airplane departed the alleyway flight 1101 could taxi to their gate. The flight crew reported multiple announcements were made reminding passengers to remain seated since they were not at the gate. Believing the cabin was secure, they began to taxi to the gate for parking. Shortly thereafter, they received a call from a FA indicating that there was a line at the aft lavatory and that the cabin was not secure. The captain applied the brakes and inadvertently brought the airplane to an abrupt stop. The flight crew then received a call advising them of multiple FA injuries. At the time of the event, FA-A and FA-D were assisting a passenger who was vomiting profusely. When the aircraft came to an abrupt stop, FA-A sustained an injury to her wrist, while FA-D was thrown to the floor and hit her head. Upon being notified of the injury, the flight crew requested to have emergency medical technicians (EMT) meet the aircraft at the gate. FA-A and FA-D were transported to local hospitals where one was diagnosed with a minor wrist injury and the other a serious head injury.