Today I was having pain on my left nipple I squeezed it and noticed a greenish fluid I am not pregnant and my last pregnancy was 6 years ago what can this mean?

There are many, many possible issues. Some of them are minor others can be serious. It is time to see your doctor. You also did not tell your age.

I am 28 years old

You need to see your doctor about this. As Fr_Chuck said, many things...some serious, some not. The only way to know is to get properly diagnosed and we cannot do that.

I am 28 years old

Okay thank you

Please see your doctor. This could be something as simple as a hormonal imbalance or infection, or something as serious as cancer.