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Allergies and intolerances
It certainly could be but only a doctor can tell you. That said, because my DS is tall I had to badger and badger more than one doctor to get a referral and even then it took a review visit to be believed.
It will start with a blood test but DS is a different child GF. He has colour, he put on weight, he came out of his shell and his confidence grew.
He also had random fevers and I would see pain wash across his face but he would deny any discomfort. I think he was just used to feeling rubbish all the time.
It all started with his poo which made me dread going out of the house because I would have to bring toddler wipes, spare pants and do my best to clean up the loo.
I would say the smell is also the biggest marker. I know instantly when he's had something he shouldn't by the smell when he goes.
I wish I had connected the dots early but I didn't.
Of course it could be something else but it doesn't sound normal or behavioural to me and ultimately you know your child better than anyone. I ended up making a private Paediactric appointment on the basis that the doctor would have to give me more time as I was fobbed of by GP when I told them how worried I was about anemia and was told there was plenty of iron in bread. Oh the irony.
We're back with the NHS now but it has taken 18 months to get to where we are from when I first asked to be taken seriously.
And he gets ill far far less these days. He was the permanently snotty kid and had so much time off with various viruses. Oh and random vomiting. I didn't realise how constantly ill he was until we came out the other side and he's not needed a day off and how little his brother has been ill in comparison. I believe it was all connected to how malnourished he had become and how difficult his body found it to fight bugs because it was too busy fighting itself. Sorry for the ramble but I could have written your OP once and now I have answers.
Whatever the cause it needs identifying. I'll say it again, you know when something isn't right with your children. 
Oh and he is IGA deficient which means the screening test can't be accurate and there is a higher prevalence of IGA deficiency in Coeliacs. And it was still an uphill battle!
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