Is it possible to make yourself get sick?

My sister has major anxiety problems. but she has the fear of throwing up. she did the thing 2 night that u do right before u throw up, but she didnt throw up. is she sick or is it in her head. she gets so worked up about it. i did the same thing when before, but i had the stomach flu, and i didnt trow up the first few times, but then i did one time. is she sick or is it in her head? please help! i feel so bad for her!