Breathing difficulties & pain with my HRM - is it just me?

I bought a heart rate monitor recently and was really excited to start using it. I've tried a few times since then and have found that it's really affecting how I feel when I exercise in a negative way. It is fine when I'm walking or not doing anything very strenuous but during my spin class, I feel like my heart is getting little electric impulses, am finding it difficult to breathe and it's causing me to feel like throwing up. I can just about manage it during spin but it's much worse during combat or kickboxing classes.

This only happens when I use my HRM - I have no issues when I'm exercising without. I thought that perhaps I had put it on too tightly around my chest so then I loosened it - it helped only a little and then meant that the strap kept getting twisted. As you can imagine, this affected the reading e.g. it was telling me that I had burned only 80 kcal after a hour long kickboxing class (which just can't be true given I burn more in my 15 min walk from home to the tube station!).

I want to keep using the HRM to get accurate readings but using it at the moment means that I feel sick and I don't enjoy exercise as a result. I've had a search on the MFP forums and I can't see any posts about this. Is this just me? Does anyone else have similar problems? Any advice on what I can do to make it better?