7 month old baby taking cephalexin for uti for the past 7 days. has severe rash. is she allergic?

Possible: Yes it could be possible usually rash due to allergy to an antibiotic appear within first 24- 48 hrs, but a late reaction is still possible. Also keep in mind we cannot rule out a viral rash without looking at the rash. Let her doctor see the rash and decide wether it's more likely an allergy to antibiotic or just a viral exanthem. Hard to differentiate sometimes.

Maybe,maybe not: Kids can have an allergic rash to an antibiotic or it could be unrelated. If the UTI was not confirmed by a culture, the fever that may have triggered exam and treatment may have be a virus that has come to the skin. It would be best to have the rash evaluated by the doc who started the medicine.