A chance of pregnancy?

I went to the doctor and did a urine test and it was negative for pregnancy, but I ended up having sex a few days later with no protection. He didn't finish and I wiped up the pre-ejaculate before he entered. We only did it for a few minutes before I decided against any further. It's been five days since then (I am on antibiotics & probiotics for a virus I have.) and I wiped in the bathroom and noticed the TINIEST spec of blood, I kept wiping and made sure that it was coming from my vagina. It never got any bigger then the size of a dot from a pen but it never went away either. I was wondering if this is even possible to be implantation bleeding or it's my antibiotics making screwing my PH balance (which im sure it has considering my discharge is insanely different.) and because its so raw down there that I made myself bleed? I am still a week or two away from an expected period and unfortunately we had sex during the week or so of my supposed ovulation. I am definitely going to get a pregnancy test but to ease my troubles until tomorrow I could use some answers.