So I was wearing my baby yesterday using Tula and after a while, I noticed he had red marks behind his knees. Is there anything I've done incorrectly?

How old is your baby and do you still need the infant insert? Please ensure that your baby's legs can still flex freely and not hyperextended. If the marks are similar to pressure marks from wearing socks, it's fine and they will go off after a while. However, if the marks are showing blood circulation being cut off, please take your baby out immediately and ensure that the carrier is suitable for the current stage.