Thank you for writing.
We cannot say if the muscle pain in your back is from your surgery but it is a side effect of bypass surgery, in general. Pain can last some time. You should have a post-surgery plan where this can be addressed including possible physical therapy - depending on where you are in your recovery - we don't know if you just had it or it has been weeks/months.
Either way, this pain needs to be addressed by your cardiac team.
We wish you the best in your recovery.