Lethargy Without Sugar

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So, I'm trying to lose the baby weight of about 30 pounds and I'm exclusively nursing my 6 month old (he's not eating solids at all yet). I only recently tried to start losing the weight though. Ever since I had my son I have an obscene sweet tooth. So I've cut my calories AND I'm working out at least 4 days a week for 30 minutes each on the treadmill. This week I did an hour per day, except for Wed & Sat. Mainly because of being tired & sore muscles. But Saturday was worse and then yesterday was terrible. All day long I had no energy whatsoever!! Completely lethargic. I even had extra green tea for caffeine and it did nothing. Last night I decided I was going to have a "free meal" since I was craving bad things and Sundays are usually the days I eat something bad if I decide to do so. I made homemade popped corn with real butter & salt on it and my son had made a toaster strudel and I smelled it and knew I had to have some too so I made 2 of those. These are the frozen poptart looking things with fruit & icing. I ate all this for dinner and I felt AMAZINGLY better after!!! I decided early on not to do "free days" but would have a free meal if I felt the need maybe once a week or once every two weeks. I'm kind of surprised because I eat Jif peanut butter for breakfast every morning and some type of fruit. Anyone else had something like this happen? I have no idea how much sugar I'm supposed to have every day but apparently it hasn't been enough!