Now I expect to be flamed for this, and for it to seem really silly to lots of people....
.... but I was diagnosed with IBS around 4 years ago now after being terribly ill on and off for a few months.
I was told to keep a diary to figure out what 'triggers' caused me problems. After around a year I worked out what foods and drinks caused me to be poorly.
I love milk, will drink 1/2 a pint a day on its own or have it with cereal, I rarely drink any hot drinks. However, I soon realised that if my milk was not REALLY fresh (used same day opened or day after) I got terrible stomach pains. So even thou it's more expensive I tend to buy a pint at a time for home use and I try to avoid drinking milk elsewhere.
I nipped in to see my sister today after work, she is well aware of this strange issue of mine. She asked me if I would like a drink. I said I fancied milk but if she didn't have 'fresh' milk i would have some water. She went to the kitchen and came back with a glass of milk claiming 'I have just opened it' I drank it and soon after left.
About an hour ago I started to feel really funny, stomach pains and grumbling pains
I couldn't figure out where I had gone wrong and rang my sister to check on the milk she had given me. She laughed and said it had been open a few days but it was fine so she didn't think anything of it and thought I was just paranoid 
AIBU to be upset with her? I didn't say much to her on the phone other than explain that I do think that could be why I may now feel poorly tonight 