Hello everybody! Water retention issue

Hi everyone,

I just joined myfitnesspal. My name is Jonathan, I have lost about 90 lbs in about 9 months and I am trying to lose these dang last 5 lbs. Because of my personal life, lifestyle and work I can only eat and drink at dinner, making it hard for me to diet properly. I believe these last 5 lbs (that I've already lost - for a total of 95 lbs - and sadly regained) are water, as I was soooo dehydrated that I put 'em on in a couple of days just by drinking water, a couple of months ago. I usually eat salads, some meat, little carbs and very little sweets and sugary foods (just when I feel craving them or debilitated). My weight is fluctuating terribily lately, 3-5 lbs a day if I'm not being careful. I run 3/4 times a week (usually 1h, but when I can 2hrs). Do you have any suggestion on how to lose water retention for good? Also up to a couple of months ago I was pretty regular (bm wise) and now I keep getting costipated for 3-5 days.
Anyway that's all for me! Thanks!