Hi there, Thank you for choosing HCM for your query. I would be glad to help. As you mentioned that your daughter had UTI and she was treated with septra and she has developed hives at the end of her treatment and these hives seem to be all over her body now and are changing colors and are joining together. It is a good thing that you are giving her benadryl for the same. Your daughter maybe developing a reaction known as erythema multiforme. This is a hypersensitivity reaction that has been associated with certain infections and certain medications. It is unpredictable as to who will get this reaction. This reaction can take various forms and can be dangerous in its serious form. It usually starts with a rash over the body which may look like hives and then take the form of target lesions and in its severe form can affect other areas of the body like mucus membranes for example, the mouth/eyes/genitalia. The description of the rash of your daughter that it is getting purplish in the middle of her hives and are connecting together is worrisome and gives the suspicion of this condition. Please donot delay for her to be seen by her pediatrician or take her to the ER immediately to have her examined to identify this condition or to recognize its progression on time. Based on their finding, they may suggest to get her admitted for closer observation and therapy accordingly. Hope this helps. Regards, Dr. Sumeet.