When the surgery is done, you are placed in very abnormal positions for relatively long periods of time. Some folks seem to have repercussions from that later. It usually shows up in the back or chest, and occasionally the shoulders. Muscle pain is not unusual after OHS.
If it's neck-and-shoulders, I would consider if you've been in an unusual position during recovery, either for watching TV or being on the Web (such as head propped up). If it's reasonable in your circumstance, I would suggest ensuring that you get enough movement: head rolls, shoulder shrugs, etc. While you may not be a candidate yet for a lot of arm exercises, you can still swing those arms a bit as you walk, and impart some fairly normal motion to your upper body without stressing your mending sternum.
I believe recovering natural motion is the best overall recovery strategy for your upper torso. In its own way, it's at least as affected as your heart by this surgery.
Best wishes,