"Tummy ache" refers to a discomfort or pain in the stomach, often caused by problems with digestion or eating something that doesn't agree with you. Full definition
I suffered with dreadful acne and was always exhausted, bloated with tummy aches and constantly getting bugs and viruses — I was just always run down. (deliciouslyella.com)
It's about processing your food a little bit more efficiently so maybe you don't get tummy aches or bloating, which a lot of people actually have. (thesmarterscienceofslim.com)