Pregnant?

On my last period, my boyfriend and I were fooling around, and he rubbed his penis on my vagina(on cycle day 4 or 5). He insists that he didn't have any precum, and never inserted it into me. I know that technically, there's a low chance of me being pregnant (my fertility window started on my 12th cycle day), but I'm worried because my periods are really irregular. I did get my period, but it wasn't heavy like it normally is. It was only heavy at night, and some clotting. Cloud this be implantation bleeding?