text: are you a fitness expert?? i am a woman 34 and 211 lbs.  other then my weight i am in good health.  last night i increased my time on the treadmill to 40 mins of fat burning and this morning i feel a little shaky and my heart is beating a little faster then normal.  does this mean i should not go back today?? i really want to. i'm not an expert but i did take classes in college for diet and fitness. i agree with the other poster's answers on slowing it down and taking a rest, there is one thing i'm wondering and if i knew i could answer your question more elaborately. did you feel nauseous that morning as well? when you wake up in the morning, generally that is when your blood sugar is at it's lowest. perhaps you had abnormally low blood sugar that morning. a rapid heart rate and shakiness are infact signs and symptoms of low blood sugar. did you feel better after you ate? this is quite normal. well i am not a doctor but this is my educated guess. good luck!