It depends what changes are actually going on in your cycle as to whether or not you need to take action. Do you track ovulation using BBT and CM? What day of your cycle are you typically ovulating and how long is your luteal phase? (Remember, ovulation can only be confirmed by BBT) if you aren't already tracking this way, then I encourage you to start. It will take three cycles to see a trend. How many cycles has it been that you've noticed the change? Have there been any changes in lifestyle or diet around three months before this change? Antibiotic use? A new streetlight outside your bedroom window? Anything at all?
I don't think a shorter cycle is necessarily a reason for concern. It could just mean you are ovulating sooner. If you weren't ovulating, the cycle would probably get longer, not shorter. But you could ask your doctor to test your AMH if you are concerned.