Am I not producing enough?

Prior to the past two days my baby would eat off one boob only for 20-35 minutes and that would be plenty for her. The last couple of days she will start getting frustrated, pulling away from the breast and making noises like she's not getting milk anymore about 15 minutes in. So today I started offering the other breast when she did this.  She latches right on for another 10 minutes or so. She's 4 weeks old. We have been cluster feeding almost everyday in the evenings and like I said one boob used to be enough. Last few days she's been giving us bigger breaks at night. 4-5 hrs. Any thoughts or advice??