Pumping/supply issue?

Jennifer
I am 3 weeks post partum, however my daughter arrived 3 weeks early. Due to issues with latching we are currently exclusively pumping while allowing my daughter to gain weight and reach maturity under the direction of her pediatrician and our lactation consultant. I am hoping someone can let me know if this happened to them... Starting two days ago I noticed my milk supply has cut in half. We are supplementing with formula already, but am unsure as to why it has done that. I just started taking fenugreek capsules and have started eating oatmeal daily, but am unsure how long that takes to boost supply. I am pumping every 3-3.5 hours (after the baby eats) and continue that schedule throughout the night as well. Any advice on what to do? Is this normal? Would love to hear if anyone else has experienced this before too!