What Causes A Large Bump With Liquid Discharge From Right Nipples?

Hi, I have a pretty large bump, it seems, behind my right nipple. It only hurts whenever I put pressure on it, but I squeezed a couple of times and a very tiny amount of clear liquid came out. Whenever I was around 12 years old, both of my nipples had a little bit smaller bumps behind them but they went away after a few months; and I also never thought to squeeze them, so I'm not sure if any liquid would have come out at that point. Also, I am 18 years old now. Is this a problem? If so, how serious?