It doesn’t happen every time but if I’m exercising enough to elevate my heart rate high enough, it’s like my brain tricks itself into thinking I’m anxious, when really it’s just normal feelings from working out. This only happens at the start of a workout for me. For example, I went on a hike the other day and the beginning was a steep incline so my heart rate spiked and my breathing increased. I started to think I was anxious which made me start to panic a bit and want to stop. Once a few minutes had passed I was okay and by the end of the hike I felt great!