Period 3 days late then had lots of brown mucus discharge followed by 2 days of red bleeding with small blood clots and tissue. whats this?

It depends!: If you are sexually active, whenever there is a change in the nature of your period, it is wise to check a home pregnancy test. Follow the instructions exactly, as to when & how to do one. Be sure it isn't expired. If it is positive and you have bleeding it is important to get seen ASAP. If you have pain,& bleeding, go to an ER. Otherwise, see if the problem occurs next period, too. If yes, to MD