Anal sex can sometimes act like an enema and cause diarrhea and a lot of wind within the first day (sometimes 2), especially the first time. If there is
no blood in your stools,
no fever, vomiting, or massive pain you should be fine and it will go away on it's own; no cause to worry. However, if you do notice blood in your stools, get a fever, or start vomiting you should go to the ER. It's theoretically possible she caused some internal damage.