What you want to be doing is lowering your weight but upping your reps. General rule would be doing 3 times the amount of kg you're doing. So if you lower it down to 10kg then do 30 reps, that'll help build muscle stamina as well as toning and fat burning but not build too much mass. 1kg is about 2.2lb if you're using pounds.
As for increasing weights, the moment you can easily surpass 12 doing one weight then you should increase it then. But you shouldn't go too high, keep your reps between 8 and 12. If you're hitting 8 or 10 and then struggling with 11 and 12 then you've got the right weight for you, if you can't even get to 8 reps then lower it.