Building muscle requires an increase in calories; that is, to gain weight you must eat more calories than you burn each day. But if you go overboard and eat too much, you’ll kick-start the fat-storing process. Obviously, the more calories you eat, the better the chances of maximizing the muscle you’ll build. Everyone has a limit as to how much muscle they can physiologically build in a month's time (for most men this is around one to three pounds with the rate slowing the larger you get), so if you want to max that out, you need to eat enough. You think you're going to gain three pounds eating a measly 100 extra calories a day? That’s not going to happen; it takes about 2,500 to 3,500 calories a day to build a pound of body tissue, depending on whether this is fat or muscle mass.
You should be lifting the amount that’s right for you today. In your next workout you should be trying to lift more (even if you can’t do more weight, try doing one more rep, or with less rest between sets) than you did last time.