Your doctor should know about that. It's normal. Taking a deep breath - expanding your chest - puts pressure on your vagus nerve. And that slows down your heartbeat. As a matter of fact, one of the treatments we used when I was a paramedic on an ambulance was what's called a "vagal maneuver". When a patient has some type of racing heartbeat condition, SVT or whatever, we would instruct them to take a deep breath, and press down like they were straining to have a bowel movement. This stimulates the vagus nerve, which slows things down, and may break the tachycardia. This is also why many people die a rather undignified death on the toilet. They pass out from the vagal maneuver that they didn't even realize they were doing, wonk their head on the tile floor, and end of story. We used to find a lot of patients on the floor, beside the toilet. One dude - poor guy - weighed like 400 lbs, fell off the toilet and got wedged between the toilet and the wall. His blubber oozed its way down around things and it was like he was cemented in there. We had the firefighters smash the toilet with a sledge hammer so we could get him out. He didn't make it.
If your doctor is unfamiliar with this vagal thing, you might need to find a new doctor. He/she would be right to be concerned if your resting heart rate is indeed 120, but the fact that it drops with a vagal maneuver is nothing special.