Deworming medicine needs to be given routinely  to every child after one year of age and the dose is 200 mg of albendazole once at night time and then repeat the same dose after 14 days once this cycle should be repeated every six months at least till 5 year of age but it is advisable to give this medicine only after taking guidance by your doctor

It may be worm infestation  or a low grade stomach infection . You should give anti worm medication:  Syp Albendazole 10 ml once single dose If she is not getting better we can give something for stomach infection . If that doesn't help , get a physical evaluation by the Pediatricain who may order some tests including an ultrasound abdomen If deemed necessary