Why period? Why?

So the last few months i noticed my period coming sooner then normal. I started when i was 11, im 30 now so yea ive had it a long time now. But its always been every 28 days like clock work. And now its every 21 days. Now i know periods change as you get older but has this ever happened to anyone else? Is it normal to have them that close? (Im sure it probably is) im just curious.