A negative C-spine x ray is nowhere near enough reason to not get yourself treated. I treat motor vehicle accident patients every day and see the common errors both patients and other doctors make. The first week usually patients feel little to no pain, just discomfort and soreness. The pain gradually comes on, and with it serious disfunction. Prompt treatment of your injuries will 1) Help manage this inevitable pain and disfunction, and 2) help to minimize future residuals, ie: do you want that back and neck pain the rest of your life?
Bottom line, you injured lots of stuff in the accident, ligaments, tendons, muscle, nerves, etc. Whiplash injuries of 5mph have been shown to have long term effects such as pain and arthritis in the future. Get yourself regular physical treatment and dont settle for less.
I have clinics in Massachusetts and California, let me know if you are close by.