My retainer hurts so bad?? (20 characters)?

So I got my braces of yesterday and they gave me a retainer. I wore it almost all day yesterday (they didn't give me instructions they just said wear it) I only took it off when I ate and brushed teeth and talked, so I left it in while I slept. Thi morning I woke up with a horrible pain in my mouth and when I took the retainer out it stopped. I really don't want to put it back in now. Why did it hurt so bad today but not yesterday??