What does it mean when someone says something hurtful and then says "I didn't mean it"?

That's not cool. I think your mom may have issues to say hurtful things to her own child. I believe that people should be impeccable with their word. Say what you mean and mean what you say. And tell her that what was said, was said, and she can never take it back, whether she meant it or not.

She thinks it, then thinks she shouldn't have said it.

google "emotional abuse"

What the person means is "I didn't mean for you to be hurt by it".

you probably took it the wrong way... want kind of hurtfull things did she say?

After she says hurtful things, she probably realizes how what she said ir horrible and covers it by saying she didn't mean it. Maybe in the moment she spouts out things without thinking, but regrets it later because she really doesn't want to hurt her child.

A lot of times people say things in anger that they don't mean. I think that she really doesn't mean to say those things. You also need to watch her actions and see if she is sorry, actions speak louder than words.

if a person says something that means they thought about it before

alot of people say things without thinking first! and believe me its usually not how they feel