Hello,
Spotting is a very common phenomenon that occurs in all women. It usually starts after ovulation, it can be mid menstrual as well. But, what you have to be concerned about is your heavy periods. A plus or minus of 5 days to your expected period date is normal and accepted, as luteal phase varies from person to person.
But your heavy period has to be checked into especially as you are saying you are not on birth pills as well. Firstly, I suggest you take iron pills as there are chances you can get anemic because of increased blood loss. Sometimes because of stress, in few they tend to get irregular and heavy periods. By stress I mean both physical and mental stress. I suggest you wait for you next menstrual cycle and see if still you have heavy periods.
If you do, then I suggest you consult a gynecologist who will suggest you to go for USG.
Thank you.