ever rare steak is cooked (or rather SHOULD be cooked) in a way where inside of the meat, despite being red is heated up to the temperature high enough to kill any food born bacteria. my guess is the same as other people - plain, good, old fashioned dirt. dirty glass, plates, cutlery, dirty hands of a person who served you the food.
drink lots of coco water - better than gatorade and other artificial hydrating drinks. eat few bananas to bring potassium up to good level. chicken broth for some strength. do not, i repeat, DO NOT take anything to stop the flow
(like chip said), it will only make you worse. get yourself tested in the morning for bacteria. i am pretty sure you will be prescribed paramox (common deparasitante).