Research activity conflicts with advisor
My PhD advisor somehow decided to start her own field research in the same region where I have been working on for a few years now. 


Which could have been a win-win situation for both of you, but it seems that you and her have a communication problem.


  Even worse, once I told her that I wouldn't be able to help 'her' project (I'm busy with mine!), she started cutting me off from my resources, withholding information, etc.


So you did not have the idea to ask her how the two project can be aligned in a constructive way?


  When I consulted other professor, he said it was pretty normal that multiple researchers do similar researches at the same time in the same region. But is it common between advisor and her student as well?


Yes it is common. And in the best case groups the supervisor and the PhD student will align the research and will co-author papers on the subject in question.