Hello,
Spotting mid cycle can be due to implantation bleeding (due to implantation of the zygote after fertilization) or hormonal changes. Since withdrawal method is not very successful, so the likelihood of pregnancy is there. For confirming the status of pregnancy, I suggest you to test for pregnancy by a highly sensitive pregnancy test 10 days post ovulation with a sensitive ( 20 - 25 mIU ) pregnancy test. Also then I would suggest you to get your serum HCG levels estimated since this is more specific than urine for pregnancy tests.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Well, the withdrawal method isn't effective at all -- just for future reference. The emergency contraception can screw up your period, so what you're experiencing may be a result of that. However, if your periods do not normalize in a few months or if you begin developing symptoms of pregnancy, take a test. If it's negative, wait a week or two and test again just to be sure.