What could be some reasons for blood being stuck in my vagina?

Outflow obstruction? This depends. If you are virginal, "trapped" blood in the vaginal canal could be due to an imperforate hymen (a curtain tpye effect at the vag opening). Self examination with a mirror may help. After a cycle, there is normally some "menstrual debre" left in the canal that sheds as browincg discharge. This is ok. Douching is not recommended for vaginal health.