I don't think she trusted that fart at all - with each of her four trips to the bathroom her shit would be getting less and less solid with more and more liquid and gas. As the indigestion from the food poisoning progressed, the pressure of holding it in became unbearable and lifting her butt up and blasting out whatever was in her colon was necessary. The first second or so where it was all gas would've felt great, and when she felt the diarrhea coming it might have felt unstoppable, or that the relief right now was worth the mess later, or as she clamped down the nausea hit and she decided that washing out her dirty drawers would've been better than vomiting. Hurling would've probably caused her to pass a massive Hershey squirt anyway when her muscles contracted to force the stomach contents up.