Hi, there are very good chances of getting pregnant
when you have sex during the ovulation time which falls on 14th day of a female with 28days of menstrual cycle.
Adding to it, 2-3 days before and after this 14th day which are also fertile period
and high chances of getting pregnant. So, in your case, you had last lmp on 17-09-15. But you haven't mentioned about your normal menstrual cycle and it's regularity!