IBS is not just in your head!!! (But I won't poo on anyone..that'd be nasty)
Mine came about from being a premature baby = (in my case) very weak and sensitive stomach stomach and was aggravated by serious bouts of food poisoning I got in Zante last summer.
I was on medication briefly, tablets which help stop contractions in your bowel, but now I just try and manage it with diet.
You can't get it diagnosed without going to the doc, it's just a combination of symtoms which by themselves could be anything. I get lots of stomach cramps, weird feelings in my stomach and all sorts of other nasties which I wouldn't go into on here..but anyway it's not pleasent but controllable.
Some element of controlling it, is in your mind, having a good positive mental attitude will help, but it won't stop it altogether.