Implantation bleeding?

Hello so I had my period 10-17th this month. Although I was on my period I did have sex the 14th. My period ended when it was suppose to but for about 3 days now I have been experiencing light spotting of very dark blood. Just now I used the bathroom & when I wiped it was a very light pink & saw some drops in the toilet as well. Should I be worried about pregnancy or is this common?