clear stuff coming out of nipples !?!?

I'm 18wks pregnant and my right breast is hurting me, it's like aching. So what did I do ? I squeezed my nipples and a very tiny tiny amount of clear liquid came out! Not enough to drip down, just like a few dots. What is that ?! And is it normal ? 
I'm not leaking, it doesn't come out on its own just when I squeeze