injury from rearend accident?

[shorty]

l week ago I was rear ended while sitting at a stop light. I did go to the ER to be checked out due to the fact that I was told to be checked out within 24 hours in case something showed up later. The car really did not get much damage, impact jerked me. I did not see it coming.
By the next day I felt some burning in the middle of my chest at times. It is now a week later and I have pain throughout my entire chest cavity, also behind the ribs. Just really is starting to be miserable when even lying down. Not sure if this is from the impact or is there something else going on. I do not feel sick.
Is this typical to only feel a lot worse a week later? Should I be checked out again or wait it out?