Hello,
The light pink/brown discharge from the vagina could be due to normal shedding of uterine lining (old menstrual blood) or an oncoming early period. That is the reason perhaps it comes periodically. But you should check up with a gynaecologist to rule out any vaginal infection or abnormality. The normal vaginal discharge is not foul smell and usually clear or milky. There are certain things that may cause change in appearance of vaginal discharge are like menstrual cycle, ovulation, emotional stress, use of pills or diet.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Spotting can occur of tons of different reasons. Anything from birth control malfunction to hormonal imbalances to even pregnancy. I suggest you see a gyn if this problem keeps occurring, to rule out anything serious.