What Causes Short Periods And Vaginal Spotting?

Dear Dr. Jain. i am 32 yrs old and married for 9 yrs, we are planning to ve a baby since jan'10.i ve no history of PCOS or any other health problem. ve been taking Folic acid since jan. For past 2 months my periods last for 3 days as compared to earlier my cycle being 5 days.this time my 2nd day was normal but 3rd and 4th day was just bark brown sticky discharge.did a home preg. test but was negative.please advice what should regarding tests and what could be the possible reasons for not been successful.
warm regards
priya