Gaining weight while inches stay constant?

Hey there. This became sort of a book, which I apologize for, but I didn't want to ask an already-annoying "hey, Internet forum, diagnose me" question and get frustrated responses back about not giving enough info about diet, etc. So here goes.

I've been tracking my food pretty consistently since August 2010, but have only recently switched to MFP from Livestrong's Daily Plate. As a petite (5'2") woman who is fairly consistently active (minimum 45-60 mins of walking a day plus working out 3-7 times a week) and who eats virtually no "processed" food (i.e., no breakfast cereals, frozen meals or fast food; limited canned goods, popcorn/spelt pretzels and breads), I'm getting freaked out at what appears to be an inexorable gain over the past 3 or so weeks, and am looking for some constructive input!

I weighed myself a couple weeks ago and, shocked to see that I was up to 56.6 kg (124.5 lbs.) from my normal 52.5-54.0 (116-119), decided to begin seriously calorie restricting again after having relaxed to try to maintain. After a week using MFP with a calorie goal of 1200 net/day and being consistently "under my calories" (when exercise calories are factored in), I was relieved to see the scale down to 55.6 kg (122.2 lbs), although my inches hadn't really changed at all.

Two points:
>> With the exception of some Chinese takeout on Saturday, which I had to guesstimate, I have weighed, measured and tracked every item I've eaten for the last eleven days on here, and before that on Livestrong. Honest to goodness. No cheats, booze or snacks that did not get counted/measured and recorded.
>> On the recommendation of someone on MFP, I started following the workouts on bodyrock.tv on Thursday (I love them!). As I said above, the past few weeks I've also been very careful to walk for at least an hour a day to go to work/errands/see friends.

I expected to perhaps be up slightly when I weighed this morning, since circuit training every day has never been part of my routine before; however, I'm up to fifty-seven and a half @#~#ing kilos (nearly 127 lbs), which I haven't been at since I was still losing weight this time last year!!! This makes a gain of 10 pounds since the end of March, which was the last time I weighed myself before my 56.6 kg "oh *kitten*" moment at the beginning of this month. My measurements have not changed at all, although my waist for two months has been 3 cm larger than its lowest point ever, which I'd like to get back to.

I'm aware a furious debate exists on MFP as to eating back exercise calories, but even without subtracting them from my daily intake, I've been averaging 1400-1600 calories a day, while my TDEE, according to fitnessfrog.com, is 1872 calories a day for exercising 1-3 days a week (and even higher for 3-5). My BMR is 1376. Back over on Livestrong, I averaged about the same during the end of March/beginning of April. So perhaps not enough of a deficit for a small woman to really lose weight, but not enough to gain ten effin' pounds!!

I read a lot on here about gaining weight or plateauing while *losing* inches, but gaining weight while staying the same size is throwing me for a loop: based on my limited understanding of the topic, I absolutely do not lift heavy enough, or eat enough, to be building muscle at that kind of a rate (although that'd be awesome!). Water retention can do all kinds of whack things with a person's weight, but not, I believe, consistently and increasingly over a month.

Has anyone else had this experience? I had considered overexercising as a possibility, but in such a small person I don't think that can be the case...Anyways, thank you for reading if you got to the end of this, and any input would be much appreciated!