Weight loss red lined

I am experiencing a problem that's getting quite annoying. In June I
went on a backpacking trip at 350 lbs, while on the trip, I couldnt
eat very much, normally around 500-750 calories a day, while walking
7am-830pm for 6 weeks. I lost 50 lbs to bring myself to a weight of
300. Ant that point, when leaving, I passed out due to water loss I
had thought. Now I am home, and trying to reduce weight and gain
muscle, I am a martial artist, and think I should be ina bit better
shape. My problem is this, I have excessive amounts of energy now, my
muscle is building very quickly, but I'm having to eat about 70% carbs
(about 3 lbs) a day or I pass out. If I stop training while eating
normally my body creates fat, I can gain about 12 lbs in 3 days, and
if I start training again, in 2 days I will be back to 300 lbs and
passing out. Now, what really bothers me is I try to eat a decent diet
(spaghetti, ham sandwhiches, etc). My body stays at exactly 300, not
299, not 301, but when I redline, exactly 300, time and time again. I
can build muscle and stay at that weight, but not just loosing the
weight. Also, when I build muscle, I require a lot more food. This
seems very odd to me and I dont think it should be this way. I tried
taking a multivitamin, and for the first day or two felt great, after
that every time I took it I would get very tired and just fall asleep.
Any advice?