Milk supply

Not sure if this is the right area to post this..-

I am just wondering if my milk is still coming in or if I'm doing something wrong. I have been Exclusively pumping for my 3 week old. I honestly have not been able to tell when my Let Down happens. I also don't ever have the overwhelming feeling of needing to pump. I also cant tell a feeling of full or empty. Is this ok? Or am I doing something wrong? Is my milk still coming? I know I'm an under supplier.. only getting 12-15 ounces per day. Just kind of feeling concerned.