1st day got brown spotting then on 2nd day or 3rd had red blood only for 1 day and till now 1 week having brown spotting so please can you tell what is the reason?

You should first take a pregnancy test. You didn't give an age so that makes it tough. It could be the start of menopause if your older or it could be begining stages of pregnancy. It is normal to have brown spotting and even some blood in begining stages of pregnancy. This is caused by the implanting of the fertilized egg on the side of the uterus. It causes some of the tissue to break lose and bleed. Some of this blood stays in the uterus and comes out later which is where the brown spotting comes in. Brown spotting is old blood.