Has anyone gotten pregnant shortly after labor?

NO RUDE COMMENTS. Hubby and I have a 3 month old baby.. she was born in October. We had sex the day after my 6 week postpartum appointment.... I had light bleeding in the beginning of December (could have been implanatation bleeding or my period but not too sure) I have not gotten a full period in December or January and I took a few tests 2 weeks ago and 3 weeks ago and it was negative all the times. Is there any way i could be pregnant even with all the negatives? If I got pregnant again back after my 6 week pp appointment in November I feel as though a test would be showing positive by now. Anyone have stories about taking tons of tests and getting a negative until you were super far along