Can you just be, well, tired? From exercising? You say you are new to cycling - what about other exercise? If your body isn't used to it yet, and you're still building/toning new muscles, working your cardio system, etc, why wouldn't you be tired? If I'm not tired after exercising, then I figure I didn't work hard enough! Especially early in the season (early in ski season I'm always beat after a workout, and the harder I worked, the more tired I am).

I'm just saying, this could be perfectly normal. If you've always been really active, just at other things, then that would be different. But if you've been fairly inactive, and you just started riding pretty recently, then you SHOULD be tired. Your body needs to get used to this. I wouldn't worry just yet.