I had my last period on 07/16/2015. i had light brown discharge for 2 days and then it's getting darker. i had unprotected sex last week. implantation blood or period?

Too soon to tell: If you miss your period by 4-5 days do a pregnancy test. If it is negative repeat it in 4-5 days after that. Use first morning urine and follow the instructions for the test carefully. If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Practice safe sex.