Hi dianam, I would like to know your age and if you are suffering froom diabetes
or hypertention or are on any medication. First of all the pain felt in your stomach as you have said is probably muscular pain
that you are feeling in the upper abdominal region due to the crunches or the situps that you are doing. Secondly get your hemoglobin
levels and blood suger levels done. If you are anemic or your suger levels are low then you get these symptoms of palpitation
and nausea due to hypoglygemia due to decreased blood supply and hence glucose
to your brain, that also explains the post palpitation headache. Simple thing you can do is to keep drinking some enery drinks in between and also not overdo the exercise but to slowly build up your stamina and increase your exercise intensity over a period of few weeks. You will feel much better with these simple changes and if you are anemic which means your hemoglobin levels are below normal levels then you may need to consult your family physician about the cause and correct that. Take care and happy exercising!