Does Light Spotting Instead Of Periods Indicate Implantation?

Hello, my significant other and I were "dry humping" about 11 days ago. There were two layers of clothing in between his and my genitalia (his underwear and mine). After a couple minutes, I noticed that some of his pre-ejaculation had soaked through his underwear. I felt my underwear and noticed that it was not soaked with his pre-ejaculation (or so I thought). We stopped immediately after that. This was on a Saturday, and I have a reliable app on my phone that told me my period was supposed to come the following Wednesday--but it didn't. It is now exactly 11 days after the incident, and I thought I had finally received my period. However, it has been light and only looks like "spotting," although ny periods are usually very heavy on the first day. Can someone tell me if there is a chance I am pregnant, or if my menstrual cycle is just being irregular (which it has never been this late before). Please let me know. Also, I am 18 years old. Thank you.