Lactating but not pregnant

Last week I took a pregnancy test (my partner and I have been trying) and it told me the infamous Not Pregnant. So I'm either not pregnant, or tested too early. Either way, I've been lactating! Sometimes just a little, or I'll leak a bit, or sometimes (like today) there will be a lot of breast milk. I actually was just looking at my boobs today and hit one spot with my thumb and accidentally squirted myself in the face. Is this much lactation normal for someone who isn't pregnant, considering I wasn't lactating before?