My daughter's body suddenly got stiff. Is she having an allergic reaction?

Unlikely. Allergic reactions that are severe can result in the loss of consciousness with the body going limp. The symptom you describe is more consistent with a seizure disorder and a medical evaluation with your pcp is suggested.
Need more info. My initial thoughts are to ask for more information. It would be very helpful to know more about your daughter's medical history and a more detailed description of the episode to evaluate this with better precision.