Hello there , thanks for your query. As you suspect there is a possibility it could be implantation bleeding if you ovulated late And intercourse happened around that time although it is not possible to ascertain it. So I suggest you to take the urine pregnancy test on the morning first sample of urine. If it still comes negative, you can be reasonably be certain that you are not pregnant and wait for your proper menstrual flow to set in or take tab regestrone 5 mg twice daily for 21 days . your spotting will cease entirely during this period but your periods that follow after discontinuation of medication will be heavy and little painful and you can have tab mefenamic acid to handle both. But if your upt is positive you need to visit your gynaecologist for further care. Hope this helps. Take care.