So I was doing pistol squats and felt an unusual popping pain in the inside front left of my hip. After looking at some diagrams, my guess is I hurt my pectineus muscle?

A couple days later I tried pistol squats again and I got the same pain on my inside left front hip. I don't know what I am doing wrong or how I am hurting myself. My other leg is not having this pain.

Does anyone have an knowlege to bestow upon me? Are there specific warm ups or stretches that I need to be doing before I do these squats? Is there a specific form for this type of exercise? I want to know how to not injure myself like this again.