Ive got a red rash which become really itchy all over & really awful belly pain cant wee or poop. my legs keep shaking & feeling sick and dehydrated ?

Concerning symptoms: Skin rash is consistent with hives; and when associated with rapid onset dizziness, facial/lip/tongue/throat swelling, shortness of breath/wheezing, GI distress/diarrhea/abdominal pain/nausea, weakness, near passing out...then severe anaphylactic shock must be immediately evaluated and ruled out. Recommend emergent evaluation regardless if known trigger or allergen is recognized.

Sounds serious: have yourself urgently evaluated. Your symptoms are worrisome and can be the result of anything from viral infection to organophosphate poisoning. With a good history and physical exam, an emergency physician should be able to take good care of you.

Viral infection?: But you really need to consult a doctor ASAP especially if these symptoms have been present for more than a couple of days or getting more severe.