I have had a UTI this pregnancy. Dr found protein and something else in urine and then asked if I had any pain in the back and when I said yes and showed him where (exactly where you describe only not just on the left) he said it was a UTI. After antibiotics and an increase in how much water I drank it was gone. He also said my nausea was more then likely from the UTI and he was right as soon as the antibiotics kicked in the nausea pretty much went away.

If your really worried then go and see your midwife but I would give the antibiotics time to work.