Irregular menstruation cycles can be concerning, especially when your usual cycle length changes significantly. It's common for factors like exercise to affect your cycle, but if you are worried about the possibility of a false negative on a pregnancy test, it's best to take the test with your first morning urine as it is more concentrated. Waiting a few days and taking another test may also provide more clarity on the situation. It's always best to consult with a healthcare professional if you have any concerns about your menstrual health.