Implantation bleeding?

I started on the 31st. I had bad cramping about an hr or 2 after I started but it stayed light. N it never got heavier. Today would be the 4th day. But the bleeding already stopped yesterday? I'm so confused. It was so light I was comfortable wearing a thong. Last night I didn't have a pad on or a tampon in n nothing came out the whole night. N my periods r never this short. N to begin with I was 4 days late? I have never started on the 31st before. Can someone help me? I've never had a period like this before. Could it be implantation bleeding?