PERIOD OR IMPLANTATION

Today I was changing my pad and I felt like I was going to pee and so I wiped and I had a huge blood clot. This is the first day of my period and so I wanted to know if blood clots that are pretty big would be a sign of my period or implantation bleeding? Because I've been worrying about being pregnant these past few weeks. My period is pretty heavy so far, the blood clot just scared me a little bit.