One of my breast are leaking milky white stuff and when I feel my nipple it feels like a bump is in my nipple I am not pregnant what should I do?

Probably you have an infected milk duct-how old are you? As there may be a different diagnosis i.e if you are going through puberty, it may be a temporary inflammation/hormonal, the lump, is it hard and sore? We need much more information-it may be that you need antibiotics-either way you need to see a doctor.