To be sick as a dog means to be very ill, but in this context the person is not vomiting and is instead healthy.
Context: "Three hours later I was on hands and knees under the cotton trailer, very healthy and not feeling nauseous at all."
Hypothesis: Three hours later I was on hands and knees under the cotton trailer, as sick as a dog.
Based on the context, is the hypothesis true, false, or unknown?