What Does Nasal Drip In Back Of Throat While On Bactrim Mean?

I started with stuffy nose 3 days ago . 2 days ago i went to see a doctor , he told me i had a uti Nd prescibed me bactrim. I been taking it as prescribed . but i feel shaky and the post nasal drip in back my throat. Im not sure whats causeing me to feel like this. I have bad anxiety and being sick is not helping my nerves.