What Causes Chest Pain Upon Deep Breathing?

I m 66 years old have atrial flutter, was cardio verted a year old, which didn t work, and have taken warfarin for about a year. I also have had high blood pressure since 1988 which is well maintained. Woke up this morning about 4 AM to go to the bathroom and felt this aching in the center of my chest. It s not a sharp pain but a dull pain that I ve never experienced before. Breathing deeply brings the ache, normal breathing is ok. I did nothing yesterday to warrant the ache (lifting anything or any physical strain) so I ve ruled out that kind of a pain. Any recommendations or possible causes? Thank you . Loren