The hives could be from the antibiotic. It is possible to develop a drug allergy at any time. My husband had used a particular antibiotic for years and was fine with it, then all of a sudden he could no longer use the antibiotic due to hives.
Also, some information I've read (don't remember where anymore) that suggests thyroid disorders can be associated with an increased risk of developing allergies.
My Mom's allergies went away when she took kelp; it helped her low thyroid to become normal.