Spotting 4 days after period..

So my period was on the 11th..it comes on the 11th of every month so I'm regular... it lasts 7 days, so it ended on the 17th... anyways, I have had sex on my period.. from the 15th to the 20th... just an hr ago, I noticed some pink spotting.. very light. What could this be? This has never happened to me before.