Period two weeks early

Hi everyone, this is my first post on here so excuse me if I do anything wrong!
I've been on the combined pill for just over 3 years now and I have never had any problems.. I didn't see my boyfriend for 3 weeks over Christmas as we are long distance. When we were reunited we had sex, and the next day I came on my period 2 weeks early. This has never happened before, it's like a full period with lots of clots! I'm now on the 4th day of my period.
Any ideas why this could be? It scared me quite a lot!