Lots of female clients come to me with mid & upper back muscle tension. Sometimes this can be caused by incorrect bra fitting (esp. over-tight bra straps) over many many years. What once may have been the right bra for you, may now have changed (as our bodies change) so its well worth checking out.
There are local bra fitting services out there but here is a link to a great fitting guide if you prefer to do it yourself.
*Fellow blokes - please pass this on to any women you may think would benefit.
Thanks & hope this helps