Why Does My Heart Rate Elevate Even With Mild Exertion But Stays Normal During Extreme Stress Or Pain?

Hi, I had a question about heartrate during stress. My heartrate jumps to 144 and makes me ill and it is being looked into. I call them attacks they are sudden but end within 30 minutes and can be caused just by walking upstairs. I understand that stress can increase heartrate but there is one thing that I think may be important but seems to be overlooked. In times of extreme panic and pain like having to go to emergency by ambulance my heartrate is in the 60 s. I would think if it s my body coping with anxiety that in a heightened time that a low rate is a symptom.