In that particular context, where a person has a stomachache and goes to the toilet but there’s nothing, I think IBS is a very common culprit. Among other symptoms, other than what I mentioned -- there are two types of IBS. You could have the constipation-predominant type, or you could have the diarrhoea-predominant type, which means that you get a lot of constipation or a lot of diarrhoea.
And then you have a third group, which doesn’t have any of these, but have the symptoms of what you described. They feel like they need to go, but they go to the toilet and nothing comes out. Sometimes they pass out mucus stools, and sometimes they have tummy pains, sometimes there are regular positions that may be better after they go to the toilet or after they let out some gas from their bottom.
So in the age group of the 20s and 30s, I would say IBS is very likely.