I’m sorry for the late reply.
Apply some ice packs to the swelling area for now. If the swelling doesn’t subside, an appointment with a vet for a differential diagnosis is required in this case. In general, slight swelling like this is usually caused by an infection(parasitic mostly) or allergies. However, a confirmative diagnosis can only be made at the clinic. ER is not necessary as long as she is not showing any anaphylactic reactions such as difficulty breathing and spreading of the hive(swelling).

Red swollen welt on the lip, whisker area