What Causes Thick Liquid Discharge From Nipple?

hi I'm a 37 year old female, yesterday I noticed a small dot of a thick liquide coming out from my left nipple, when I sqeezed it some blode cam out i went to see my doctor today and when he squeezed the nipple no blod came out but just some uncolored liquide. He asked me to do a mamographie and some blood tests which i will be doing tomorrow. I'm not taking any medication but I'm having my period. based on this information can you please advise if this is something serious? Should I be worried about breathet cancer? what to you suggest that I should do in order to take all available precautions?