How much sodium are you eating? I would assume quite a lot since you are getting a rapid weight gain.
Another thing, if you are going to use Cozaar, you most likely need to take it twice a day. It is a very weak ARB with a short half-life. Diovan seems to be the most popular ARB. I believe Benicar is probably the most potent. Micardis has the longest half-life (24 hours). Though, I remember reading on here that some folks were having problems with 24 hour control using Micardis.
FWIW, I got absolutely zero BP control with Hyzaar (Cozaar + HCTZ) 50/12.5. However, Benicar 20 mg (no HCTZ) is giving me excellent results. My average BP before was 135/85. Now, it is averaging 116/66 on a low sodium diet. If I eat too much sodium (even for 1 day), I will put on 2-3 pounds and my BP will go up about 10/10 (losing about 1/2 the drug's potency). This will last for about 3 days after which things will settle back down.