How many hands did you play in those 2 weeks? I can't imagine it's very many. You need a LOT of hands (as in millions) to start narrowing in on your true winrate. Could just be variance. I'm a winning $100NL 6-max cash player (or used to be at least, before Black Friday), and I had multiple 100,000 hand break-even stretches. Sample size is a really big deal. It's really impossible to determine if someone's a winning player or not unless you have millions of hands on them, and even then, variance will take its toll. I know the math's been done on 2+2, but the point is, you need millions and millions of hands before variance no longer plays a factor in your winrate.
Also, are you multi-tabling? I find that I tend to lose when switching from MTT's (where I'm usually firing up a bunch of tables and mindlessly grinding until I get deep) to Cash games, where you really need to focus on no more than 4 or 6 tables. 6 is probably pushing it. 4 or less is good because it allows you to get a "feel" for the game flow, who's on a rush, who's tilting, who's trying to present a certain table image, etc.