Did I read that you're eating 1800 calories a day? I would say if your riding 100-150 miles a week, you need to eat quite a lot more than that. If you're an avid cyclist, you're body I'm sure is burning a lot more or at least should be burning a lot more than that each day, unless it's in starvation mode, as others have suggested. Watch out for overtraining too. I'm trying to follow a pattern of 3-4 weeks of progressively harder training and then one week of light or minimal riding. You can do other things to improve fitness while giving your body a break from the intense work that is bicycling. I like to go rollerblading occasionally. Others choose jogging/running. My body thanks me for those light weeks with improved riding when I do get back on. I do still ride on my lite weeks, but instead of 4-5 days of 20+ mile rides (I can't fit in longer rides than this most of the time), I might ride 3 times in a lite week and make the rides 13-14 miles each. There is great benefit to changing up your workout a bit on occasion.