For most people, glucose will spill over from the blood into the urine when blood glucose levels reach 10 mmol/l as the kidney cannot manage to reabsorb all the glucose when the blood glucose levels get this high. This is an average value, though, and some people will get glucose in the urine at lower blood glucose levels.
If a vaginal swab has come back negative for infection, you might like to consider whether you are having a local reaction to bubble bath, shower gel, toilet paper, condoms, contraceptive gels etc. and avoid all potential irritants to see if the burning sensation improves. If not, go back to your GP.