I have a constant pressue that is pushing my vagina open. Now that I have my period my tampon is coming out from it. What's causing the pressure?

More info. There's definitely a condition called vaginal prolapse where the vagina falls down and out. How old are you? Have you had vaginal births? See a doctor for this as there are remedies. A pessary. Surgery.
See a doctor. You may have bladder or uterine prolapse from poor pelvic muscular strength, but you also could have a tumor that is pushing things down. You need to be examined to know what is going on.