i posted this on the calorie thread also............ ok here you go. your cells hold more water in anticipation of needing it for the longer runs or harder workouts(it is actually more technical than that but you get the point). you body is an amazing machine you have to feed it for it to be efficient. 50%+/- carbs is good they will provide the needed glycogen you body needs and that is where the water come into play, your body automatically stores water with glycogen. also make sure your protein is also in a 25%+/- range to repair the muscle damage you do while training. you want to go 40-30-30 if you are not working out.
a typical woman(150 lbs) needs around 2000 calories per day, figure you burn about a 80-120 calories per mile so and all the other stuff you are doing look what you need to fuel your body just to maintain and keep it healthy.
you are probably putting you body into starvation mode. starvation diet does not mean the absence of food. it means cutting the total caloric intake to less than 60% of what the body requires. it slows you metabolism so you can't drop weight as easy or even gain and you body does not get enough nutrients.
just an example if you need 2000 per day to maintain weight. you only eat 1500 which is ok but then you burn 1000 you end up with a net of 500 calories. with a net of 500 your body is starving for nutrients, any and begins to hold onto everything you put into so you can actually gain weigh on 500 calories per day. good rule of thumb is never go below 1200 calories net. if you have been doing this to you body it may take several weeks to turn it around to get back to normal.
i fought with anne(miamia7) about this when we first started spark and once she started eating she went from 200 to 125 in about 8 months.
load up on fruits and vegies for your complex carbs, good fats and protein