Your original sentence "It was a very learnful experience" would normally be phrased "The experience was very educational" or "It was a very instructive/informative experience"

Educate, inform, instruct, teach, have similar meanings and can be substituted for leerzaam when translating to English. Instructive is likely the one that best fits your intention. The context of the sentence would clarify what was learned.

Leerzaam in English would be educational, informative, instructive.

Instructief and informatief in fact are given as Dutch synonyms for leerzaam

As so often happens, the word does not have an exact counterpart in English, but rather a short list of words with similar meanings that cover various specific uses of the word being translated.

Upset stomach

a disorder of digestive function characterized by discomfort or heartburn or nausea

Edit — As seen from the comments, this is actually an informal term for Dyspepsia.

Dyspepsia, also known as indigestion or upset stomach, is a term that describes discomfort or pain in the upper abdomen. It is not a disease. Dyspepsia is a group of symptoms which often include bloating, nausea and burping.

Diarrhoea often follows Dyspepsia. So unless you need to provide specific details about your stomach condition, it's best to use upset stomach. I wouldn't like to hear from an employee the words like Diarrhoea, the runs etc. The imagery is off putting.

Note: Diarrhea isn't always contagious. The infectious diarrhea is called Gastroenteritis or stomach flu*.

_{^{*Info gathered from various posts on this page. comment1, Malvolio's answer, comment2, Martha's answer, comment3, Dave's answer}}