The last couple times i've had sex i've puked afterwards and then once during. what could be the cause of this? i've taken a couple pregnancy tests and they came back negative. i am also on birth control.

Mind: It is hard to know why it may be happening. Could it be that in your mind you are associating sex with something revolting enough to make a person vomit? If it persists, it would be prudent to seek counseling, as it is not likely to be due to a structural issue. Unrelated to your question - If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Practice safe sex. Get HPV vaccine.