Is her poop green? Sometimes even frothy? Does she have a LOT of poops and pees every day? Could be milk oversupply - which can feel unsatisfying to a baby because she's only getting the watery foremilk and gets filled up on that but stays hungry for the more substantial hindmilk.
askdrsears.com has a great tip on how to solve it. Worked for me, and it was very easy!
And if this is not it: keep google searching all of her symptoms till you find a match - and how to solve it. Reflux (could be that it doesn't come all the way up, e.g. "silent reflux," or food allergies could also be the cause. Don't buy the "some babies don't tolerate breast miilk" theory. If more than, I don't know, .5% of all babies had this problem, we wouldn't have made it this far as a species! It's not like our ancestors had formula, LOL. Your breastmilk is all that your baby needs. You might just need to work on something, like your feeding pattern (if it's an overabundance - aka foremilk/hindmilk imbalance) or what you're eating. Cut out dairy and see if that helps, and the other major food allergens (wheat, soy, peanuts, citrus). What about caffeine? Chocolate, coffee? Cut 'em out!
Don't give up. Be an advocate for your baby. Breastfeeding is too important to give up without a big fight first, and your baby is in pain and needs you! Ask everyone, be very persistent. Call the La Leche League in your area. You can find them through a quick search. You can even have one of their volunteers come out to your house to do an assessment, and their help is always free.
I'm sure you'll find the answer soon. I was feeling pretty hopeless, myself, just a few days ago, after weeks of this (since soon after her birth!), was starting to supplement with formula - same deal - and thinking it just wasn't going to work out, but finding the right answer made all of the difference. My baby and I are really enjoying life now! You and your baby will, too.
OH, AND - DON'T MAKE HER "CRY IT OUT." It sounds like you're not doing this, but if tempted or advised, keep this in mind: never make a baby cry anything out. They need you. They're crying for a reason, even if it's "just" comfort that they need. For a baby, that is a very important need, too. Babies eventually outgrow many of their needs, or the intensity or frequency of them. Be there for her while she needs you. It will get better! For more information on "crying it out" read Dr. Sears, either at askdrsears.com or any of the Sears series books. It just makes sense.
L.
PS for any fussing, I've found that:
Running the shower water - on cold to save energy! - is very soothing. I must have walked two miles in my little bathroom already!
Also, getting into the tub helps - babies love water, you probably noticed - and helps with nursing, too when she's fussy. It can be very dangerous if you're by yourself, though, so if your husband's off on duty, and grandma's not around to help, you might want to skip this one. It's safest if you get in and someone hands you the baby, and you hand them the baby while they're holding a towel when ready to get out.
Try the "colic dance" - any set of steps that works for you, but at 60 "beats" per minute or so, and in all three planes: up and down, back and forward, and side to side. Just read that in the Sears' Baby Book, works like a CHARM.
"Bicycle" her legs; lets out lots of gas.
Also, inserting a lubricated rectal thermometer or pinky finger into the anus, lets out gas - and sometimes some poop. Just be very careful to never force anything where it doesn't seem to budge. Probably just need to wiggle the thermometer a little.
Try going "SHHHHH" really loudly in her ear. Very soothing, like in the womb.
Or swaddling. I got these last two from THe Happiest Baby on the Block - get the DVD!