Milk fluctuate?

Does the amount you get fluctuate? I’ve been doing good pumping. My son has to get formula and breast milk for a bit because he had high bilirubin levels and they just kept climbing no matter what we did. So I was pumping. Well I started him on nursing again Sunday. So I nurse him and then pump afterward then the next feeding he gets formula and I didn’t pump and then just kept going back and forth. So I pumped about every 4 hours and got 5-7 oz. well last night I only got about 4 and I’m pumping now and it’s looking the same. I know if he nurses I should expect less but this happens when I’m just pumping and giving him breast milk from last pump session in a bottle. Don’t understand why it’s going down some. Am I going to have to start pumping every 2 hours