My sense is that you're not taking in enough calories for your level of exercise. I have two sports nutrition books that I really like, both written by registered dieticians. They are down to earth books, may be worth a look: Eat Smart, Play Hard : Customized Food Plans for All Your Sports and Fitness Pursuits by Liz Applegate, and Nancy Clark's Sports Nutrition Guidebook by Nancy Clark. Both are available at Amazon.com.
I just did a quick search on the web and found a "healthy body calculator" that seems pretty reasonable. It's on the site Ask the Dietician, which is also authored by a registered dietician. The calculator is at http://www.dietitian.com/ibw/ibw.html
Just out of curiosity, I ran through the calculator a couple of times. With what I would consider a normal work day - which includes 1 to 2 hours of moderate exercise, I came up with a caloric requirement of 2230. I suspect serious cycling falls somewhere between moderate and heavy exercise. If I put in 4 hours of moderate exercise, my calorie needs come out to 2610, and if I put in 4 hours of heavy exercise my calorie needs show as 3040. So likely somewhere between 2610 and 3040. But even my average daily rate, assuming my sedentary work (I sit at a computer all day...) and an hour or 2 of exercise comes out to a lot more than the 1200+ calories that you say you are eating. (Even with no exercise I still came out to need 1800 calories. Oh, and of course height & weight plays in here too - I'm around 5 ft 5 inches and weigh about 115 pounds.)
--- Denise