If this is new swelling and pain, you need to call your doctor.  In general, symptoms following surgery should be gradually getting better.  Notice I said "gradually."  Following a mastectomy, it can seem like you are making no progress at all until you are six months or a year out and realize that you have had fewer pains this month than last.  I still have muscle spasms twelve years later that cause sharp pain.  However, if you are having new and/or increasing pain, that is not typical.  Swelling three months after surgery probably also needs to be checked out.  

Finding a comfortable sleep position is a long-term issue as well.  If the heart pillow is working for you, it may be your friend for life.  I have a squishy tube-shaped pillow that helps me.

When you call the doctor to discuss the pain and swelling, ask about a referral to a physical therapist.  A good therapist can help get those muscles active again, which should help.