ObesityHelp

I am almost a year post RNY and about five pounds from goal.  I have been working out consistently since eight weeks out.  I am testing for black belt in four weeks, after five years of training.  I had a month long stall in December on weight loss, then I lost eight pounds in January when I started doing 2 a day workouts four days a week and one on Friday.  (Increased my cardio and kept my weights at 45 minutes a day with trainer.)  I consistently eat between 1500-1800 calories a day, I am 5'10 and I my weight last week was 160.2  (lowest yet.)

I gained back  eight pounds seemingly overnight.  It dropped down three pounds one day, back up the next, and now I have been sitting at five pounds gained for about five or six days.

It isn't muscle - obviously I didn't put on five pounds of muscle overnight and my scale says I have LOST muscle mass and gained fat mass.

I know your muscles can hold water when you are training hard. Is this a possibility?  5-8 pounds worth?

I am not freaking out, really - but I do want to get to goal weight, and this is the first time since surgery I have gained weight that isn't a one or two day water fluctuation of a pound or two.

Thanks for any advice.  I read here all the time but rarely post because my connection is so slow.