What is causing my diarrhoea?

I got up late on the 12th of June 2011, about 1pm. I didn't notice that I felt unwell at the time. About a half hour later I had a Sunday dinner/lunch. This consisted of steak and potatoes. After I had strawberries from a package. I did not wash the strawberries but always eat them this way. The steak was not well done and a bit moist and I'm not sure but maybe it was faintly pink in the middle. Afterwards I had some chocolate bar and drank some warm milk and water as I'm trying to give up tea. All this stuff I am used to eating. Soon after I had diarrhoea. I thought nothing of it but since then it started occuring after all food. On the 17th of June after eating a brown roll with tuna and some chocolate I decided to stop eating and just take water. I am still experiencing diarrhoea but it is more watery. The area below my navel has a cramped, bloated type feeling like I need to pass wind. This has accompanied the diarrhoea since it started but initially only seemed to be more to the left. This feeling is not particularly uncomfortable but is noticeable constantly. I have gotten diarrhoea in the past but thought nothing of it. Usually it would be the morning after a night out or after eating too much Chinese food but even sometimes after these it wouldn't happen at all. So it is not common for me. I have no allergies and can eat everything. I drink at least a half pint of milk a day and can have all other dairy products. Also I have never had an allergic reaction to wheat or any other cereal or grain and eat products made of or containing these daily. I am a light drinker and would have about four pints on a night out. I have only drank twice since mid March to mid June. The last time I drank was the night of the 10th and early hours of the 11th of June and the beer I drank was out of date by four and a half months. I drank quite a lot that time, about twelve bottles which is probably about eight pints. I am not sure could this be linked to the diarrhoea. All the 11th I did not notice any symptoms and ate meals without problems. I usually exercise, running and bodyweight and am fit enough but the week I got the diarrhoea I was not and was eating quite a lot over the weekend and had takeout on the 10th. None of my family members or relations have any problems regarding stomach, bowel etc. e.g. Crohns, IBS, etc. What is causing this? It's hell and has to stop soon :-( Is there anything else I can do in the meantime? I have seen a doctor, she gave me a tube to place a sample if it went over one week which it will and advised against imodium saying if a bug were the cause imodium would keep it inside.