insurance/belly sickness

My four month old is recuperating from a bout of vomiting and diarrhea really just one day but 5 days of flagyl
his malaise it seems has taken his energy hes just laying around we will go back to vet if his energy stays low
I bought him pet insurance from http://www.trupanionpetinsurance.com/
I told the salesperson also when asked that he had the diarrhea and vomit
would such a common disorderr be construed as a preexisting condition?

Yep, it is a pre-existing condition since he had this when you bought the policy. If he is cured, and it happens again, and they find it is unrelated to this, then it will not be considered pre-existing anymore.
Is he eating/drinking/using the box?
Try to lift his food dish a bit, to slow him down on eating. Also you might try a probiotic/prebiotic to calm down his digestive system...