My boyfriend trying to put in his penis from the back but went in my vagina a bit by mistake,a drop of blood showed after 10 minutes. could it be the result of losing virginity even if before that the tip has gone in several times?or is it an injury?

Penile penetration of the anus or vagina ends any claim of virginity. Bleeding from the vagina after penetration accompanies tearing tissue. If there was an intact or residual hymen, an erect penis will tear it. If it was not intact, it may represent other torn tissue. If it is minimal and stops soon it may self heal.

A slight injury could do that specially if the hymen is not completely gone.

There was injury to either just the hymen or vagina. It should heal on its own. Your behavior is putting you at risk of pregnancy and STDs. Get HPV vaccine. Use contraceptive all the time, every time. Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines