Hello,
Thanks for posting your query at DoctorSpring.com
Functional cysts generally are asymptomatic.
Since you are sometimes erratic with your birth control, the altered menstrual bleeding could be due to that most probably.
Do not worry about it, as your pregnancy tests are negative.
Once in a while, some factors like stress, weight changes, travelling, drinking, smoking, late nights, erratic eating, exercise, change of climate etc can cause this.
Just have an internal exam and a pelvic ultrasound scan in addition to testing for thyroid hormone levels.
All the best
Please feel free to discuss further.
Regards.