I am not a doctor (nor do I claim to be, even on TV
) but in my experience, with weight loss and weight gain, the fluctuating estrogen levels will wreak havoc on my cycle. I would say any drastic change in your weight and exercise levels will affect your cycle, including the length, amount of flow, etc., unless of course, you are on some type of BCP or hormonal treatment for birth control.
At my highest weights, my cycle actually stops for a few months, then comes on for months
at time, which is a nightmare. This has happened to me twice and both times it spurred me to #1) See the OB/GYN and get a Rx for BCP and #2) Get back on the ball and eat healthy, exercise and as a consequence, lose the weight. It eventually goes back to a more normal (whatever normal is for you) pattern. If it continues to get heavier or longer than is normal for you, I would definitely recommend a visit to the doctor.