Lower back associated many diagnosis
With the history and investigation you did ,It seem no issues related to you kidneys , the pain is from 6 months and lower back , we need to rule out issues related to spine, so need to examine you with detail history and may be do some radiological screening too.
You are young so may also be due to postural issues
So better to get examied by a clinition.
Next Steps
vist the nearest orthopedics or general physician for clinical examination
Health Tips
do keep taking pain killers, find out the problem and try to solve it .