Is Passing Of A Tissue Like Discharge In Every Period Normal?

I usually have really mild periods. I notice that with every cycle I pass tissue like substance that is roughly 1 inch in length and as wide as a pencil. The tissue is usually a red/purple color. It happens every period and I even use it as a mental timer that my cycle will become lighter after it is passed. It only happens once with every cycle. I feel a slight pressure/discomfort when I am about to pass this tissue. Just wanted to know if this is normal. I am sexually active, not on birth control. I have been with is same guy for the bast 2 years and we both get tested regularly and are negative for any STI's.