4year old uti now a skin rash

[Inneedofaholiday2017]

Diagnosed with a uti yesterday and given antibiotics - she's had 3 doses and I'm not sure she's shown much improvement and thus evening she's come up in an itchy rash on her torso and legs - is this normal-unrelated to uti? Her legs were a bit itchy yesterday before she started antibiotic.
How quickly should she be feeling better? Should the antibiotics have started to work by now?

What antibiotic is it? My daughter had an allergic reaction to Penicillin V and came out in a rash. The Drs put her on another antibiotic. Hope your little one feels better soon.

Trimethoprim... sounds like I might need to go back to drs tomorrow to switch antibiotics! Thanks