Symptoms of a peanut allergy are severe enough that you probably already know if you have one, but eating too many peanuts can cause tummy troubles even if you don't have an allergy. Peanuts, like many beans and legumes, contain phytic acid and lectins. These compounds make peanuts difficult to digest (think of it as the peanut's defence mechanism), and eating too many peanuts (or spoonfuls of peanut butter) can cause unpleasant symptoms like gas and bloating.
Almond butter, walnut butter, or tahini (sesame seed butter) are lower in lectin than peanut butter, so they make for a good substitute. If you really want to test the effects of phytic acid on your digestion, soak and sprout nuts at home or purchase sprouted nuts from a health food store.