Don't feel guilty for not exercising when you're sick; you need to get well before you can push yourself into even better shape!
As far as the gain, I know it's almost devastating. It does make some sense though if you were increasing calories to around 1800 from around 1200. How long were you eating the 1200 calorie diet? Moving up will cause a shift. The thing is, when you stop losing at 1200 you'll still have to slowly increase your calories anyway, and that will cause a tiny gain as well (which once you're at adequate maintenence you can promptly lose again - but its a tough slog of a game really). So the real question is whether you want to do that now or later.
I'm don't mean to be telling you how to do it, just that the only question is whether you do it now or in a month or two (or three, I don't know).
Oh yeah, the legal disclaimer (lol): I'm not an expert by any means