This season only started getting a lot of flats in my back tire (riding the same Trek 4300 mountain bike for past 6 years and never had this issue). In last 7 days had 3 flats - put new tube yesterday and had a flat today again! Using Lyzene pump to inflate to 40 psi. Inspected the tire and there is no evidence of any protrusions. The puncture in the tube is on the inside - a tiny slit. Inspected the rim as well and no evident sharps there - I have a double wall back wheel.Thinking maybe the tire (original since new) is worn out and is causing this problem. Appreciate feedback. Cheers!