Bactrim is usually prescribed for UTI’s (urinary tract infections). Even though a rash can be a side effect from the med., it also can appear with a viral infection too.  If you haven’t other symptoms, I would discontinue the med & contact her MD ASAP.

If you have  fever 103 or higher (this could indicate a bacterial infection or worse, a systemic infection from the initial bladder infection). Be on the safe side and call yours  MD (even if you have bactrim with no prescription). Take Motrin to lower the fever (that’s what the ER would give ) and make sure you drinking adequate fluids