High heart rate

Lately my heart rate has been very high in the morning. Every morning about 45-50 minutes after I get up, my heart rate skyrockets to somewhere between 120-140 beats per minute when the resting rate is somewhere around 70-75. I always thought it was just anxiety because I start feeling anxious, tired, shaky, and my breathing is labored. I often have to sit down because I feel so tired. My heart rate usually returns to normal within a couple of hours. Any ideas on what this could be?