Trying to conceive. Is unable to detect?

Hi doctor,I am 25 years old. I got my first cycle when I was 14. I get my regular cycles in 26 days. But from past 5 years I always get brown spotting before my period starts for a day n then get very less bleeding for just one and half day. But now from past 2 yrs I get brown spotting for 2 days n then I get very less flow for one and half day like I jus change 3 pads a day with each pad half filled. I consulted a doctor n got lots of tests done and all my reports came normal. And she said it is normal and nothing to worry. My free t4 is 0.89, prolactin 11.83 and tsh 1.74. I am trying to conceive from Jan n every month I get brown spotting followed by flow after 3 days. I just wanted to know if this pattern of cycles are making me unable to conceive or am I going through some infertility problem.I consulted a gyn n she prescribed duphaston 10mg taken twice before 5 days of period And folic acid from 2 months. Still no improvement. Is there any problem which my gyn is unable to detect?