Hello ma'am and welcome.
Sexual intercourse does attribute to a slight change in hormonal
levels, but the activity in itself could be the cause for the bleed.
I would recommend a visit to your doctor who will conduct a physical examination. There have been many cases of initiation of the monthly period immediately after or during sexual intercourse and this is considered normal. But as I said, it is best to meet your doctor to make sure all is well.
Hope this helps.