Milk duct.. missing??

Hey guys!!! Okay, so me and my SO have been trying to convince since July or so. And the passed few days my breast have been really sore along with some other symptoms. My SO felt bad so he decided to massage them for me, all of a sudden he stops and grabs my left nipple... and it goes flat, like the milk duct is really thin... but my right nipple felt normal, we could feel it no problem. What does that mean?