I Am Lactating. I've Not Been Pregnant, I'm 18 Years

I am lactating. I've not been pregnant, I'm 18 years old, I had blood work to check every hormone, liver kidneys and even an MRI and everything came back normal. My dr. Now wants to run test and get a breast ultrasounds done. Then if that's normal he wants to send me to an OBGYN. What could this be? I haven't been able to get pregnant but my blood work is normal.