Everytime my child gets a cold she gets red very itcy (almost mosquito bite type) spots on her lower back and legs. what could it be?

Perhaps these are: not colds but actually allergic reactions. Then, these may be hives (the itchy rash) and the congestion/cough may be part of that allergy. Benadryl (diphenhydramine) may help for both, but please speak with her pediatrician to better diagnose and treat this problem. If you are a Prime member, you also could consult one us through there.