Thank you for reaching out and asking my opinion. It sounds like the bleeding pattern, pain, and timing of your bleeding is very different from normal. And it seems like you know the typical spacing of your menses if you were 15 days late. Even though your home pregnancy test is negative, it would be prudent to have a blood pregnancy test done to ensure you are not pregnant; on some occasions, a urine test may be negative but the blood will be positive. It is unlikely for this to be implantation bleeding as the timing of that bleeding is approximately 7 days after ovulation.
Hope this helps!Take care and best of luck!
-M. Dayal, MD