I have left lower back pain after i started lunges and squats. what should i do?

Activity modificatio: While the symptoms are going on, hold off from doing these lungs/squats till pain resolve. Take some nsaids (if you have no medical contraindications). You likely overexerted yourself, and sustained some lumbar sprain/strain. If things don't start feeling better, seek help from your doctor. Starting physical therapy will be helpful if the symptoms continues beyond a couple of weeks.