Baby just won't sleep!

My one month old won't take anymore bottle and her diaper has been changed. I've held her and held her and she still won't fall asleep. She keeps doing this and sometimes will be up for hours at a time (she's been up for up to six hours before! ). She just cries and cries the whole time. I keep getting so frustrated that I just lay her down and let her cry it out. I've taken her temperature and it's normal. What else can I do?? I need time to myself to sleep and relax! I just don't understand why she keeps crying.