The only thing I can find is that this is an early sign of pregnancy. Is it possible that semen was transferred from my mouth to his, to my vagina, and I got pregnant?

Hi qrious... welcome to AMHD,

No, you would not become pregnant in that situation.

There are a number of possible reasons for the spotting. It could be due to ovulation, due to a period that is about to start or recently ended, due to hormonal birth control use, a fluctuation in hormones, could be due to irritation of the cervix, etc.