Hi thanks for asking question. You have not mentioned which antibiotic you were used for stomach infection. Here it could be proctitis like inflammatory condition.Examine your anal area for fissure or redness.proctoscopy done for detailed visualisation of anus. Meanwhile follow below instruction.... -more water -fruits more -green leafy veg taken more -avoid spicy and fatty food for few days -avoid smoking -stool softener can be taken for few days -avoid junk foods Brufen tablet taken for five days. Warm water sitz bath useful. Mostly with these measure it will recover. If still problem then detailed investigation needed by surgeon with USG and physical examination. Take care. Dr.parth