Is it possible that I could be pregnant?

I am a week early in starting my period - could this be implantation? It is very light, but it feels like period but not as heavy. I am 43 years old, lesbian and I had never had sex with a man, so this is new to me. I had intercourse on the 8/16 and I had just started my period. I normally have a 28 day cycle. I started my period 9/8. I feel pretty foolish for not knowing. Can anyone help me? Should I take a pregnancy test? I know the chances are not likely, but I want to know.