Have you recently picked up the volume or intensity of your rides recently? A couple of times this season I've been training too much (too long, too hard, too frequently). When this happens I find that I can't get my heart rate up to where I expect it to be even though I'm feeling ok otherwise. For me this situation signals muscle fatigue and a couple of days of complete rest have helped get me back on track.

Have you noticed an elevation in your resting heart rate as well? This is also a good measure of overtraining.

As the others have mentioned it is also a good idea get checked out by your doctor since this change happened relatively quickly.

Hope you get it figured out soon and that all is well.

- traveller