I had the worst case of gastro when I was PG and it was from both ends I was flattened for 2 days so I'm really feeling for you.

Braxton hicks can come on when you're feeling tired or run down so it could be them. Get yourself off to the Dr or ring the midwives JIC.

Keep up the fluids as you don't want to dehydrate. Be wary of a headache as that is a sign of dehydration.

Hope you're feeling better soon