FTM pumping output question.

okay so i’m almost 4 weeks PP and i breastfeed my daughter on demand whenever she needs to and pump in between sessions if possible.

she eats a lot, always has enough dirty diapers in a day but i feel like i’m not producing enough milk!

i try and pump and get 1 MAYBE 2 oz from both sides per session!

and i can only pump for about 30 seconds and then my milk stops.

does anyone know why my milk stops or do you have any advise? thank you so much, i really appreciate it !