Is it normal that i feel the need to cry after i help a person?

That's tender. Try to focus your efforts though.

This is 'normal' I would say. When you are helping people it can be a very emotional experience. Perhaps you are reflecting on your own need for help and the understanding of how much it can mean to someone.

There is nothing wrong with crying unless you cry really often and in the wrong situations. Just keep your emotions in check and try to understand them.

Yes. It's absolutely normal. It's called compassion.

maybe it's because you have fulfilled a purpose and now you feel empty?

not sure if it;s normal, there's probably a reason to it. Perhaps you should ask someone who knows psychology?