Dear Josh,
It is rare what you exprienced. Scientifically writing, air directly from an environment is a foregn matter to the large intestine. And most foreign matter in the body cause di-ease. It is nature of large intestine to accomodate with certain amount of gases produced out of digestive processes. So, air ingested by you through anus should not harm you. Moreover, it should serve a medicinal for not allowing anarobic germs, if any, to grow in the large intestine. However, if one is suffering from digestive disorder with upward APAN VAYU flow (beltching or air in stomach) and colic pain, he/she should restrain him/her self from contineuing the practice.
Vijay Singh Gusain N.D.D.Y. (Naturopath), M.A. (Science of Living, Preksha Meditation & Yoga)