Was it a whiplash type injury ?
Even the strain/impact from the seat belt or air bags can bring on neck pain or muscle spasms.

The seat belt may have affected your lymph nodes and caused some swelling? in the arm pit area - lots of lymph nodes there- that cans affect the breast tissues also.

Here's a sticky post from the TOS forum that is kind of a crash course in TOS and therapies-
http://neurotalk.psychcentral.com/post388-1.html

some of the therapy and anatomy info will be of help even if you don't feel TOS fits for your symptoms.