IBS is a 'diagnosis of exclusion' in that you cannot have specific tests for IBS. You are diagnosed with IBS if the doctor cannot find a cause for your symptoms with all of the tests for everything else.
However, if you haven't had a colonoscopy or x-rays to exclude bowel polyps, coeliac disease, ulcerative colitis, Crohns disease, diverticulitis and a load of other conditions (your symptoms don't sound like a load of these already- they are just illustrative of how many different conditions are excluded) then there's no way that a doctor can tell for certain what is going on in there.
The bleeding could be just due to haemorrhoids, which would be very easy for a doctor to diagnose, but it could also be a sign of much more sinister things, so anybody bleeding in this way is advised to see a doctor specifically for this symptom
. The bleeding could be unconnected to every other symptom and the doctor needs to know that you are bleeding and start an investigation of what is causing it.
In your position I'd go and see your GP and tell them that you're still having problems with this and ask them for a referral to a dietician to help you work out which foods are helping/causing your problems. I'd also ask for something to help with the constipation which, if the bleeding is caused by haemorrhoids, won't be helping them at all.
The important thing to realise is that for some people a high fibre diet does not help IBS at all and for some the fibre is a trigger for IBS in itself. I know someone who has dreadful IBS after eating any sort of wholemeal bread, lentils, brown rice, chick peas, garden peas and all sorts of other high fibre foods. It makes organising a diet rather tough for them and they have a much less varied diet than is acknowledged to be healthy, but the alternative is having crippling gut pain and being unable to leave the house for weeks at a time.
A dietician should be able to help you both work out what foods are contributing to your problems and how to change your diet to be as healthy as possible whilst avoiding these foods.
Oh- and prunes aren't rank