I been taking smz - 800 mg for 9 days for a uti. i started to have a rash on arms & legs . is it bc on the antibotic im takin?

Probably allergic: Sufla-trimethoprim is one of the most common causes of allergic drug rash. Stop the antibiotic and check in with the doctor who prescribed it ASAP. (If s/he isn't available during the weekend, someone probably is covering.) In any case, most UTIs are treated only 5-7 days, so it may be safe to stop treatment entirely.

Drug rash: It's certainly possible - as SMZ is a sulfa medication - and about 10% of people have side effects from this class of meds. Usually a drug rash will be most prominent on the chest/trunk, rather than the limbs- but of course rashes can be variable. 9 days seems like a lot to take for a urinary infection - I would DEFINITELY talk to the doc that prescribed the medication. Do it today, before worsens