This is known in our family as water-bottom......
I'd slightly disagree with Jno, as individuals differ.
Can you bear to fast? Avoiding food will mean the bacteria have nothing to feed on so exit. Keep drinking water and flat lemonade if you decide to try a fast.
First aid to keep you hydrated is simple - a handful of sugar and a pinch of salt in a pint of water, stir briskly and drink. You don't have to drink it all in one go but stir it well before you have a sip.
I'm not in favour of immodium - if what's in there wants out, don't try to keep it in!