Implantation spotting?

So I last was intimate on the 7th of December, and ovulated on the 9th of December. My period is not due until the 23rd, but all last week and this week I've been having mild on and off cramping, and then today I began spotting. I usually never get spotting before my period is due, and never usually cramp until the day my period starts. I also have mild breast tenderness. I'm not sure if the spotting Im having could be implantation bleeding or just the start of my period earlier than I expected. Help!