Implantation or period??

So my period was due on the 16th so it is now 4 days late. Just today I went to the bathroom and there was what looked very very very light blood, but only when I wiped, it was only once and hasn't happened since. I've been have lower abdominal cramps since Sunday on and off but today they have been slightly worse. I have never been pregnant before so I'm not sure if this is just leading to my period or if it is implantation. Any advice would be greatly appreciated, thanks