My Niece Has Been In Hospital Treated For Cellulitis Of

My niece has been in hospital treated for cellulitis of rt leg. She was d/c on cephalexan and doxycycline and had been taking them without incidence. This afternoon she took them together and now has hives and rash over body. She took 3 Benadryl and I advised her not to take any more until she talked with her Md.
Do they interact with each other?
YYYY@YYYY