Why Are My Menses Light After A Non Productive Intercourse?

Hi I am 17 years old, and my boyfriend and i were fooling around the other day. We decided to just try sex for 2 seconds, so he cleaned his penis off ( he had not previously came) and put it in and took it out, thats it 2 seconds. The next day my period came. Although it was heavy -ish on the first day and i had cramps like usual it seems to have only lasted 3 days. Every day after that it got lighter and lighter. My period is usually 5 days long. We took the morning after pill just as a precaution although he did not cum or pre cum or anything when it was 2 seconds long. I am just wondering why it is so short and so light. I have been stressed wondering if my period has come, and i had recently started to ski again.