When my partner ejaculates inside of me, i get a burning, raw, numbing sensation in my vagina three days after intercourse? why only when he cums?

Burning: Couple possible causes and the first thought is infection. Trichomoniasis especially will cause intense vaginal symptoms after sex. So, time to see a gynecologist about that possibility. Other STDs and infections need to be ruled out also. Some women do have an issue with semen almost like an allergy. Again you need to see a GYN for evaluation.

It is possible that: you have an allergic reaction to his semen. This is a known phenomena. It is a good idea from a safety standpoint for you to be using a condom, so try that and see if you have the same sensation after intercourse. If you do not, then you have found the problem.

Semen allergy: You have likely become allergic to semen which is not likely confined to just a specific partner. You need to practice safe sex especially at your age. If you do plan to get pregnant in the future, consult an allergist to consider getting desensitized to seminal fluid