Welcome to Healthcare magic dear parent.Tummy ache and tightness has many reasons.I will go with least scary and most common one.It may be due to colick.For it s remedy you can use anti-colick drops which are available over the counter.Drops containg alpha-galactosidase (read:ga-lakto-see-days) agents that help in milk digestion are also helpful.Also try leg pumping exercise.They are really helpful.You can learn them from youtube. To rule out other scary causes of tight tummy watch for these danger signs: very bloated and shiny abdomen,Blood in stools,persistent or yellow vomiting,constipation,inability to pass gas.In which case immediately take your child to nearby emergency department. Wishing good health to your kid and good luck to you