What Causes Irregular Periods And Lethargy While On 'nuvaring'?

Hello, Im 43 years old. I have noticed that whenever Im on Nuvo ring my athletic performance decreases, Im a an accomplished triathlete and the only reason i went back on Nuvo ring this fall was that my body fat got so low that I started missing my periods and mensis became very irregular. I have noticed that I became sluggish and my performance decresed. Is it possible that nuvo ring causes it or maybe other factors such as stress, etc..i appriciate any advice, thank you