I have a couple on each nipple. I believe that it is actually "milk" you are squeezing out. If you were pregnant it would be more liquidy and more of it would come out. It is just one of the milk ducts from your nipple but not quite in the right spot.
I have not had a doctor tell me this, but my husband who has a child says his ex had this too, which squirted when she was breast-feeding. Also, I have had other female friends confirm this.
I personally enjoy squeezing white stuff out of mine, but I am strange that way.