I'm having light pink to light brown spotting on and off for 3 weeks. Not pregnant but trying for a baby. What can be the cause? No period for 4 months

May be infection . You should have an evaluation. Bleeding between cycles can be caused by an infection like chlamydia. Other causes can be due lack of ovulation, uterine fibroids or hormonal imbalance such as an underactive htyroid. You can not conceive if you are not having a normal ovulation each month.