My daughter was put on Singulair 2 weeks ago after discovering sh...

My daughter was put on Singulair 2 weeks ago after discovering she is allergic to 4 kinds of trees that are pollinating in the fall and a certain mold that grows in corn and bean fields. Everything was going well until she got an ear infection and was put on the antibiotic amoxicillin which she has used off and on when she's had ear infections over the years. She then broke out in the hives. She has never had the hives before. I'm guessing it could be a combination of the two drugs. Why aren't doctors aware of this before prescribing this medication?