I agree with Basil that, in most everyday situations, you would avoid going into the precise details. Among friends, family, colleagues and so on, I'd say something vague like 'upset stomach'. For example: 'Would you like some orange juice?' 'Umm, no thanks. I've actually got a little bit of an upset stomach, so I'll just have some water.' Your friends and family would not want to know any more details than that! You would only use 'loose motion' or 'loose stools' in a medical context. Doctors and nurses use these terms - they would not be appropriate other contexts. It isn't that these terms are not polite, it's more that they are rather too detailed and unpleasant-sounding.

In ordinary conversation, "stomach infection", although a bit inaccurate, is more polite than referring to the actual symptom/infection (diarrhoea/gastroenteritis?) which raises unpleasant images in people's minds. If you are seeking medical treatment, you should be as explicit as required to by the medical professional.

if u reffering to diarrhea, u should say loose stools. I have loose stools.

"loose motion"? Can you give an example of the sentence? I've never heard of "loose motion".

What is the context that you would like to use the word? Could you use it in a sentence so that I could understand what the exact word should be?