My daughter, who is now 6 weeks, also crys quite a bit for apparently no reason. In western country, this is called "colic" or "prolong crying" and it's still a great mystery in baby care. Don't worry, if you have brought her to see the doctor and the doctor said she's perfectly healthy, then she will outgrow the condition by 3 months. For the time being, by trial and error, you may find some way to soothe her. Eg. my daughter's crying can sometimes be stopped by listening to boring noise, which reminds her of the days in my tummy.
Try to go to some websites like www. babycenter.com or www.americanbaby.com and search for "COLIC", you will learn more about it.