Your body would swell? I'm not sure what you mean....like you literally got bigger before you lost some fat, or you had a reaction, or....?
If you want to lift weights in order to improve your appearance and lose bodyfat, then a high rep/low weight scheme isn't necessarily going to work for you. If you don't feel as though you're putting a lot of effort into your rep schemes (i.e. after 12 reps you've barely broken a sweat), then you're not really doing anything productive for your muscles other than burning a few calories.
If you want to stick to the 12-rep, 3 set weight scheme that's ok, as long as you're using a challenging weight. Not only that, but once that particular weight becomes to easy you have to up that weight.
No, don't worry, I'm not turning you into a bodybuilder.

But if don't start challenging yourself with your weights then you're just "spinning your wheels".
Remember:
muscle growth =
high metabolism = more fat loss