Could it be IBS? You say you remember it starting months ago, had you ever had it before? Or did it come on after some sort of gastric upset? Does stress make it worse?
It is a miserable problem but there are a few things you can do.
Worth considering because the common fixes will not do you any harm if the problem turns out to be something else.
So try:
Small meals (about the size of a chinese rice bowl) with about 2-3 hours in between. Eating a big meal always upsets my stomach. If that is still a problem reduce the portions again. You are basically trying to make life easy for your stomach and gut. If you can bear it, sometimes a fast day, just drinking water or peppermint tea, gives your gut the break it needs.
If you're going out then until you have it under control I'd suggest don't eat for 3 or 4 hours beforehand. Our bodies are pretty simple - food in means poo out!
Diet change: (1) no or less wheat (I can eat some but more than one portion in a day is definitely a problem), (2) no dairy (3) no sugar (again I can eat some but too much is a guaranteed issue). Obviously try them one after the other and give each about a week. You'll see the difference fairly quickly if any of those are triggers. Ideally keep a symptoms diary - probably a good idea anyway while you are awaiting your referral.
Go to the health shop and get one of the multiple strains of bacteria supplements, ideally those either marketed specifically for IBS or aimed at people travelling to third world countries. Expect to pay about £20. Take this for at least three months. This made a huge difference for my daughter.
Peppermint oil capsules (e.g. Colpermin). Very good at easing cramps and nausea.
Try buscopan as well as imodium. Buscopan is an antispasmodic so it stops your gut cramping. Some people find this alone is enough.
Finally experiment with fibre. Some people are helped by adding more to their diet, some, including me, are worse with additional fibre. You can buy fibre supplements in health shops too.
If it is diagnosed as IBS (i.e. no known cause) then it is very manageable with the measures above, you just have to work through to what works for you.