Hot water bottle relieves the pain of abdominal spasm by
Stimulation of adrenergic fibers
Stimulation of cholinergic fibers
Inhibition of cold receptors
Inhibition of heat receptors
Pain from gastrointestinal viscera is carried by small C fibers. Temperature sensation from hot water bottles passes through A fibers and large C fibers. 
When these larger fibers are stimulated, they block the adrenergic small C fibers from the viscera.