What Causes Intermenstrual Spotting?

Hi Doctor,Im 24 years old,I was getting my period every month for 35 days once,last month I noticed some brown spots for almost 1 weak and later I got my regular period, it took for almost 7 days to stop bleeding and bleeding was not heavy. I consulted doctor too ,she said to reduce weight ,and from the day 1 consulting of doctor I started my exercice daily for 2 hours n burning around some 400 calories . About food also im well planned.But again this month Im seeing bit brown spot before my period........So Im worried so much again what all remedies I should take?? Is this serious problem?? Please suggest me........