Lower back pain in left side is a common problem. Before answering i'd like to ask you that how many hours you sit on chair in a day?
If it is more than 5 6 hours daily then definitely your Lower back pain is because of this.
I had been suffering from Lower back pain for few YEARS because i am a software engineer and i need to sit around 10+ hours a day on the chair.
But now i do not have this problem because i used to do one activity called Stretch Hamstring on daily basis for 20 minutes specially in the morning.
If you don't know what it is then you can refer below steps explained well here