Well, I'm not a Dr...
I think 6 weeks old is FAR too early for this. If you look around online, it seems that is a general professional consensus. She has been on this earth for 6 WEEKS, if she is crying, she needs something. Something doesn't feel good in her body, she needs food, she needs physical contact (holding,) she needs something. I'm curious, does she also spit up a lot, or seem to have digestion issues? That can many times correlate with crying a lot, especially when the child is laying down. It might be time to see a pediatric doctor who specializes in reflux, if that's the case.
Wait, do you have the Dr. that told you to put rice cereal in the bottle? I think you need a new doctor...he doesn't sound like the best. I would not trust ANY doctor that told me to do that to a 6 week old.
I will give you a tip, though. Try probiotics. It was a lifesaver, for my sister's colicky baby, who had reflux. Go into a natural food store (or vitamin store), and look in the section where they keep the refrigerated vitamins. They will be in powder form, so you can put them in the bottle. They will specifically say they are for babies. They will ONLY help your baby, in one way or another. My son has had them every day since birth. They are completely safe. They help regulate the gut naturally.