In my opinion, letting her cry is not the answer. She needs something from you. Sleeping 5 hours IS considered sleeping through the night. Breastmilk is very quickly digested and this is why she is probably waking. But don't give her anything else as it is the best food for her.
I had to go back to work at 10 weeks with my son and he was very colicky and woke alot at night like your daughter. What worked for me is that I would put him to bed in his crib and go to bed when he did. Then I would get 4-5 hours of sleep most nights. Then when he woke, I would bring him to bed with us and he would nurse off and on while cosleeping until I had to get up in the morning. This gave me some uninterrupted sleep and still let me rest when he needed me. Everyone tried to tell me that I needed to let him cry it out, but I stuck to my guns. Then when he was about a year old I tried to break him of the cosleeping by using a method developed by Elizabeth Pantley in "The No-Cry Sleep Solution". In a nutshell, I would still bring him to bed, but I would only let him nurse for a few seconds, and then take him off. If he fussed, I would put him back on, but again, only for a few seconds, and each time you decrease the amount of time you let them stay on. You have to stay awake and alert, but for me it only took about a week and then he stopped waking because he knew that he wasn't going to get to nurse constantly if he came to bed with us. I would also keep track of when he woke and when he nursed and over time you could see the intervals getting less and less. At that point I knew he was getting plenty of solid food to keep him satisfied through the night, and then I would just nurse him before I left to work and he would go right back to sleep until Daddy got him up. My son goes to sleep great now, he learned himself with time how to soothe himself, and he is confident that if he needs me, I will be there. I hope this helps. I highly reccommend that book.
I would also suggest that she could be teething, and in this case she definetly needs some comfort. When they are in pain, you have to do what you can. The pain will pass eventually and the sleeping schedule will get better.
I would also suggest trying a white noise cd or sound maker. This was extremely soothing to my son and was the only thing that would calm him slightly when he was colicky.
Good luck! I know it seems it will never end right now, but they are only this young for a short time, and their confidence and trust in you for attending to their needs when they are young will affect their trust in you their whole life long.