Hi, I Am Experiencing Constant Vaginal Tearing In The Posterior

Hi, I am experiencing constant vaginal tearing in the posterior area of the opening of my vagina. It has happened two times and both of them occur right after (or during) sexual intercourse. After the first time, I let it heal and did not have sexual intercourse for approximately 3 months. Now I recently had sexual intercourse and I felt the tear again. I don't know what is causing it because I'm young (20 years old - so no menopause) and before these two tears I have had 2 years of perfectly normal sexual intercourse. My partner has been the same all along, so I don't know what to do. What are the causes of this? How can I prevent it? How should I treat the tear?