It sounds like the med is doing its job in clearing up the infection. I wonder if you are having some irritation/side effect from the med itself, I have a vague memory of something like that happening to me years ago when I had UTIs a lot.

Can you talk to your druggist (chemist?) about it and get his opinion? In the US there are some over the counter meds you can take for the pain, I don't know what is available in the UK or if you should continue the medication.

judi