Too much exercise? (Irregular periods)

Zoe
How much exercise is too much? I started going to the gym a few months back, but my period is a week late and still waiting (and no, I am NOT pregnant).
I exercise 7 hours/5 days a week at the gym, and do 30min of gentle yoga everyday at home. I thought this was pretty mild but I did drop 6 pounds in the past 2 months, and now my period isn't happening :(
My period used to come every 5 weeks, and it was extremely light flow - was it even normal before?