My Three Year Old Wakes At Night With Tummy Pain

My three year old wakes at night with tummy pain . It also occurs in the evening - he has reduced Appatite and is pale . He is toilet training - fine at per - still not getting there for poo. We also Hav pets. I'm wondering if this wakeful pain cud be constipation because he's been holding in poo until night time occasionally (having one when he gets nappy on) but he's had bowel movements last two days during day -, or could it b due to worms from dog? There is no anus itching . He's very irritable and upset . Pain seems around his navel but it's not distended r sore to palpate ?