What Causes Sudden Inter Menstrual Bleeding?

I have been bleeding per vagina for about 3weeks now. this suddenly started a few days after my period which i started on the 17th of June 2011. bleeding is mainly during the day, usually minimal or absent at nights. its occasionally dark brown and with little clots seen. i could conveniently use a sanitary towel a day or a panty liner. am 38yrs old have always had a regular 27-28 days cycle that runs for 5 days. i have 2 kids, youngest is 3 yrs old. had myomectomy in 2005. had an hsg done already and nothing was found. had a similar bleeding episode late last year and my Gynae gave me an injection since he felt it was probably hormonal. please advice.