I don't want you to be alarmed, but I would definately get to the pediatrician just to make sure your baby hasn't lost any weight and that there's nothing medical going on. With a six week old, you can't be too careful.
Given that, it is normal for babies to go through streaks where they eat less, and then all of a sudden your baby will eat like a voracious lion. That happened with my baby and he's now a 33 pound, very strong and happy 2-year old! He still has times when he eats little, and then a few days or weeks later, will eat a whole lot.
The most important thing is not to stress, because that comes through in your body language and in your milk, if you are breast feeding. Babies pick up on your every mood. Sometimes I would look down at my boy while he was nursing, and I felt like I could see a very old wise man in those baby blues. Sometimes I would make a game out of it, especially when I would alternate with the bottle (we struggled a bit on that). I would bring the bottle just up to his grasp so he knew it was there, and then I would pretend it was a birdie and make it "fly" around him, and then bring it down to his grasp (not his mouth). He would get so excited he'd kick his feet and grab the bottle, and he'd actually put it in his mouth himself. Giving a child control over his world starts earlier than you would think. I forget how old he was, but it wasn't much older than 2 or 3 months.
Anyway, *enjoy* this time. It will be over before you know it.