You're not riding enough to be overtraining so I would look at diet. Are you losing weight. It doesn't appear that you're eating enough during the day. I would suggest closely monitoring your caloric intake and expenditure for a while and make sure you're not trying to sustain too large a deficit. You shouldn't be having more than 500 Cal/day of deficit.

Another suggestion would be to go the doctor and have some basic blood work done to rule out any potential issues.