Hi and welcome to HCM.
I'm Dr. Taimur and I will be addressing your health concerns.
I have read your question and understand your concern.
First time heavy and late menstrual bleeding may be due to
Miscarriage , if you are sexually active, ( usually associated with lower
abdominal pain and cramps)
Pelvic Inflammatory Disease ( that will have high grade
fever with it.)
Contraceptive Devices ( if you are using any)
Any medications like NSAIDs if you are using any.
So to reach to the diagnosis you need to properly examined and thorough history is needed.
Sometimes it may be just a single episode and it will normalise by itself, you don't need to worry about it.
My advise to patients is to wait for another period if it is normal let it go and if not then we will investigate it.
I hope this might be of some help, if you have any query feel free to ask me any time.
Thanks.
Dr. Taimur