I Have Had Stomach Flu And I Am Felling Very Weak.

I have had gastroenteritis ( stomach flu ) well i think i do, i seem t only be sick when i get up which is strange dont experience much abdomen pains to be honest rarely any, but i did at the start... at the start it was serious, vomiting, diorreah and constipation cramps ect most seem to have eased, my poop seems to be long and slimy sausage shaped id say hardish, i just dont get why iam only physically sick on a morning once i get up any ideas ? my gilfriend is not pregnant iv read everything on the internet i just want a answer from a profesional :) thanks doc