What Causes Cysts In The Ovary?

my LMP was 04/04/2011 so i checked my Beta-hCG-serum. it was 21mIU/ml. for further confirmation i went for USG and my TVS shows a semi solid cyts about 1.7 x 1.4cm with fine internal septation within right ovarian tissue which is compatible with ovarian haemorrhagic cyst. what can be the probable cause of this. thank you