From tons to barely nothing!!! Help

Tanya
Hi girls,
My baby boy is 26 days old, and I been breasfeeding him. My supply seemed pretty good was pumping  like 12 onz per day until today. 
I been felling flat, not like my usual in which my breasts are so full. I haven't change anything on my diet, I don't know what's going on. I have only been able to pump 4oz!
Is this something unusual? I'm getting worried please any advise is welcome!!!
Thank you
Worried Momma 😞
This is my baby boy Andres