Stuck on a plateau and gaining instead of losing

I'm on Nutrisystem and weigh, measure and record everything that goes in my mouth. I average between 1200-1250 cals/day, and I NEVER cheat even a tiny bit. My biggest indulgence is using a fruit exchange and an extra to make a straweberry sundae - 6 oz fresh strawberries, 1 packet Splenda, 2-4 tbsp Walden Farms 0 cal chocolate syrup, and about 2 tbsp Cool Whip Free. Total calories is between 75-80. I am probably on the low level of active, because I'm recovering from hip replacement surgery two months ago, but at this point I don't have any restrictions, other than sticking to low impact activities. I go to PT 3x/wk where I do 20 mins cardio and a good hour of assorted floor exercises, most of which are core and strengthening. I'm also doing light to moderate weights on my off-PT days, and try to do 20 -60 mins on the treadmill at 3% grade, 2.5 mph - 2.8 mph a couple of times a week on non-PT days. I'm 5'1", 58 and post-menopausal.

Having said that, i started Nutrisystem in May, 2011 at 237 lbs and I'm now down to 170 lbs--but I've lost ZERO for the past MONTH, and in fact, have GAINED two pounds from my pretty solid mid-January weight in the 168's. I've tried the standard plateau-breaking tricks: redouble vigilance on food intake, increase exercise, switch lunch & dinner, change the foods you eat. Nothing seems to be helping. I eat mini-meals throughout the day at no more than 3 hr intervals, more or less. I drink at least 96 oz of plain water throughout the day also, don't drink soda or alcohol. I have 2 cups of coffee/day, each with 1 tsp fatfree creamer, for a total of 7 cals of creamer, and I even measure the creamer!

Before this plateau I was averaging 1-2 lbs/week loss, probably closer to 2 lbs/wk. I had some plateaus but nothing like this, and none where I gained more than a couple of oz - usually I'd either sit at one weight, or bounce up and down in a 1 lb range.

I don't have any symptoms of starvation mode. I'm not weak, hungry or anything else during the day unless it's time for a snack, and I have that out of my planned eating.

I'm about out of ideas. About all I haven't tried is INCREASING calories, but I don't see any justification for that based on my exercise level and lack of symptoms.

HELP! Anyone ever have this happen to them? I find the weight gain especially worrisome.