One of my favorite post mastectomy yoga stretching is laying on my back on a bolster (you could also use a rolled up towel or a pillow.)
Laying on your back with the bolster or pillow strait up the spine from the base of the spine to the neck or head. Let your shoulder fall off to the side opening the chest.
You can keep your arms to the sides and work on getting them out to the sides as you get the scar tissue or the tight muscles if the are above implants. This will slowly open up the muscles and will help get back to full range of motion.