If I stop taking amitriptyline what side effect will I have? I take 10mg a day

A number of things. The condition you take it for might become more apparent. Low dose Amitriptyline is often used for chronic pain complaints, like headache, so that could become more evident. Most people take Amitriptyline at bedtime because it is sedating, so you might have some trouble falling asleep. Amitriptyline can cause dry mouth and constipation, so you could notice the reverse of those when you stop it.
None. 10mg is smallest pill available for Amitriptyline which is an antidepressant. Tapering off any medication under physician supervision should not lead to "side effects" but original symptoms that led to prescription may return. So stay in close contact w/your prescribing physician just in case you need to restart medication.