if you've been following your clinic's recommendations, then it's likely water retention from the IV fluids. That's basically salt water, and a lot of people retain that. There are some people on here who've even weighed 10 lbs more when they left the hospital than when they arrived! It can take a week or occasionally longer to work its way out of your system.
stick to your plan and the weight will come off. Everyone is different due to different factors, many of which you have little to no control over (like age, gender, starting BMI, metabolic level, how much weight you lost prior to surgery, etc). The two things you DO have control over is how closely you stick to your plan and your activity level. If you do well with those, the weight will come off, whether fast or slow.
also, the 20 lb loss you said you think you should have lost in the first two weeks - banish that thought. If you started out at 600+ lbs, then yes - maybe - but for us more "normal" WLS patients, losing 20 lbs in two weeks just doesn't happen. Most of us lose somewhere in the 15-25 lb range the first MONTH. Yes of course you will find people who've lost more or less than that the first month, but they're outliers. You're probably doing just fine. Give it more time and remember to stick to your plan! I thought i was a terribly slow loser - and I WAS a bit behind others with similar statistics (starting weight, etc), but I ended up losing over 200 lbs.