That wasn't your 6 pack bro! That was the vertebrae in your spine showing through! You gotta eat more food! I realize you aren't a large guy (yet), but the only time you should be eating 900 calories or less is when you are forced to (i.e. you live in a poor country and do not have the economicial means to buy more food) or....you're teen aged girlfriend's parents are gone for the week and you're too busy shaggin' her brains out to feed yourself. There is no other excuse!!!
Plus, with only 40 grams of protein daily, you are starving your muscles dude. Sure you're losing weight and your abs (spine) will show through, but you're losing muscle off your entire body, because it's having a smorgesborg on the proteins. (someone correct me if I have any of that wrong, but I don't think so)
You should probably increase everything you are eating.
As far as the diet coke...it's not "good for you", it's more of an empty calorie thing. The caffiene is a natural diuretic, so I wouldn't recommend drinking a lot of it while taking something like creatine. These guys are also right, aspertame is not good for you at all. When I'm trying to cut down, I get an empty fruitopia bottle, I fill it with water and then I put in a pack of Crystal Lite, then I throw in some Splenda or something. Another thing I've started doing is filling up 2 litres of water and throwing in about 3 sliced lemons and about 2 cups of splenda.
It's deeeee-licious! I just bring it in my fruitopia bottle wherever I'm going. I....am ofcourse being a hypocrit, because I am currently sipping a can of...Diet Coke...Do as I say, not as I do brotha!