Relationships cause me anxiety and nausea

I am in high school and have been facing anxiety issues lately. Whenever I get close to being in a relationship, some kind of anxiety takes over and keeps me from the relationship. This anxiety causes me depression at times and even makes me want to vomit. While a semi-relationship should be joyous, for me, it's an emotional nightmare I can't get to the bottom of. I've had this before and had to leave the semi-relationship to avoid throwing up every time I made contact with my crush. What could be the reason behind this? Is it massive nerve problems? Is it a fear of a bad relationship? While I'm not quite ready to be in one anyway, I want to get to the bottom of this for a brighter future in which I can be in one. I don't take medications and have no desire to, I have read forums, and I've talked about the issue with my sister who has faced similar problems.