Spotting after switching pill brands

Patient: I am 30 years old, and last period started on Jan. 17. I decided to switch from Marvelon (ethinylestradiol and desogestrel) to Generess and started the new pack on Jan.21 . Marvelon is supposed to be 21 days of active pills and 7 days inactive, but I only waited 6 days to start Generess. I took the third pill in my pack 14-16 hours late and continued the rest as usual. A day or two later I had sex with my husband with no backup, thinking I was covered since I missed 1 pill by less than 24 hours (he did pull out, I know that’s not reliable though). A week later, I started spotting. Now it’s 4 days later, I am still spotting and have mild cramps. I know it’s too early to take a pregnancy test because I’m not due for my next period for another 12 days. Could this just be breakthrough bleeding or is it more likely implantation bleeding?

Symptoms: spotting (4 days) – need a pantiliner
mild cramping
no other pregnancy symptoms