That's not an allergy mate, that's food poisoning.*
pint glass of water, teaspoon of salt, tablespoon of sugar. drink it. repeat with as much as you can drink without making yourself vomit from drinking too much of it.
wait about 24 hours, if you haven't stopped shitting then, get some live yoghurt, or some of those friendly bacteria drinks down you. You will, by then, have likely cleared your system of all the real nasties. The problem will be that you will have cleared out all the symbiotic bacteria as well, you need to keep your guts running well and that will also give you the shits.
If you have to, go for the immodioum, you can get a generic from the pharmacy for under half the price of the branded stuff. Give yourself at least 12 hours to clear out the nasties though. no point in bunging yourself up if you bung a load of toxin producing bacteria inside yourself and giving yourself symptoms worse than the shits. I prefer to avoid that stuff though.
*not a medical opinion, but I've seen food pooisioning enough times and I've seen food alergies of the sort that cause nausea and vomiting. My partner can vomit on an 8 foot high ceiling while lying on the floor if he's given marmite or blue cheese. his alergies have never made him shit. the foods never get past his stomach (he drinks water that he then throws up and rinses out his stomach, can't keep water down until he's washed himself out). he also gets breathing problems and a swolen and sore throat. IMO, if you have the shits, it is bacteria, not an allergen that is the cause.