Hot water bottle relieves the pain of abdominal spasm by
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.
Stimulation of adrenergic fibers
Stimulation of cholinergic fibers
Inhibition of cold receptors
Inhibition of heat receptors