Why is my 8-week puppy's sleeping heart rate low and belly hard?

This is a little lower than what I would consider typical, but this might be normal for this dog when she is completely asleep and relaxed. Typically puppies her age tend to have a heart rate fairly similar to an adults although it can be a little faster, and heart rates are usually much faster at a veterinary hospital where there is stress and excitement. You may be feeling her spleen or other normal organs on one side of her belly compared to the other. If she is otherwise acting like a normal puppy I would recommend mentioning the belly and low heart rate at her next vaccine appointment. The veterinarian can listen to her heart for murmurs or arrhythmias and feel her belly.