Hi. I think i have understood your problem.
As your both pregnancy tests are negative, your nausea and crampy feel might be due to gastric irritation( stomach upset
Furthermore your stress might add to hyperacidity
Change in the delay of periods might be due to stress,and undue fatigue due to excess exercise.
Occasionally once in a year if you get change in period dates its not that worrisome.
try out de stressing with meditation
and Yoga, do planned exercise in moderation, inculcate healthy food habits.
if your periods turn quite irregular please seek medical help