Missed period by 17 days with a BFN.

Hello! I am 25 years old. I had regular menses by far but since last month i have noticed some unusual changes in my menses. I am trying to concieve since last 3 months now. I had a very light bleeding during my last period without any cramps( which i generally do get during initial few hours of the first day). I took a pregnancy test which showed negative so i ignored the change for the time being. However, i am late by 17 days this month and again a negative pregnancy test. Doctor suggeted me meprate for 5 days thrice a day. What could be the possible reason for such a delay as it has never happened to me bfre? Also, My question is, am i still going to ovulate this month despite of the delay in my periods so that i can keep trying to conceive this month or should i wait till my withdrawal bleeding of the last month and then start trying again? I am extremely worried since this had never happened to me before. Please help.