My DS is a similar age and I just got my AF back a few weeks ago. I had the sort of symptoms you're describing on and off for weeks in the lead up to it along with occassional light spotting, so I think it probably your bod just starting to get back into the swing of things. Like you I also wondered if I could be pregnant, I even started a thread asking about implantation spotting LOL before I eventually did a HPT, got AF back four or five days later.