Are you in the UK? I am. And I understand the huge difficulty getting an appointment with your GP nowadays. It is getting very bad.
So ...anyway, I am not a doctor, I can only try to be a little bit helpful but ideally you will need to see the doc. when you can.
If it gets really bad, try ringing NHS 111. Sometimes, an alert put to your health clinic by them can be very helpful in getting to see a doctor, or having a doctor phone you.
Anyway, if you normally do have IBS, then sudden symptoms can be hard to work out....is it food poisoning, or a bug?? Or just the old IBS back again for a bad flare up. I know that one. It can be difficult to know how to treat it for the best. For instance if it's something you ate, it's best not to take immodium, and let the body handle it by flushing out toxins quickly. But if it's IBS, immodium could help!
Until you either start to feel better, get to chat with your doctor, or it just passes and goes away for now, then a very very bland diet would help.
Everyone's "safe foods" are different, but go low fibre, low residue for a day or two and see if that calms your gut.
Something like hard boiled eggs, chicken breast steamed or baked with just salt, white fish cooked the same, white rice. and if you're OK with fruit maybe small amounts of banana. Or if you have some safe veggies, a small portion of that.
You could try mashed potato or baked but don't eat the skins. And white toast, with butter if that suits you. Just drink water, or weak tea with no milk. Or maybe a camomile or peppermint tea.
See if that helps
Whatever it is, that diet will help it settle and be very easy to digest. If you start to feel a bit "bound" you could gradually introduce more veggies or fruits that normally suit you, or try a bit of wholemeal toast.
(oops...I didn't read all the posts before saying all this, so it seems you did see your doctor? And they think it's the IBS? The above is still relevant. I hope you feel much better soon and come out of that flare up.)