What are the possible cause of blood spot after a week of menstrautions ends if not using any contraceptives?

Occasional "spotting" can occur just prior to ovulation, when two eggs are matured simultaneously. The estrogen levels are elevated, and the more severe drop in estrogen can cause visible spotting, similar to what causes menstrual bleeding at the end of a cycle.

Spotting between periods is common and often has no obvious cause. But if it happens more than once or twice, it should be medically evaluated. Many causes are possible, but a common cause at your age is an STD like gonorrhea or chlamydia (but of course only if at risk, like new partner, partner with other partners, etc). If it happens again or any other doubt, get care soon.