I am sexually active. My period was due May 18th and I got it on the 8th of May and it lasted for 5 days. On the 16th I went to the bathroom and discover dark brown blood in my panties. What do this mean?

Your period may have just been off this month... the dark blood is usually older blood that your body is still expelling. See what happens next month... since you are sexually active, there is always the possibility of pregnancy and the earlier "period" may have not been an actual period... especially if it was lighter than your norm.