When I have a lot of wind/faeces in my stomach I get a wave of extreme intense tiredness and very fast heart rate until I pass the wind or faeces. Why?

Autonomic response. This may be due to a reflex autonomic feed back loop, when your large intestine gets stretched, you may develop increased sympathetic tone which leads to rapid heart rate and this leads to decreased blood supply heart heart beat... So take some deep breath and go to the toilet asap!