What Causes Vaginal Bleeding Between Periods?

Since Tuesday, Feb. 24 I got a light period and I thought my period should be coming but the same amount of blood just came, which was really light that I didn t use a tampon nor a pad. In the past week, I did use an occasional tampon. Its been three weeks and still there s no sign of stopping. I m only 22 years old and this never happened to me and its getting very worrying.