Feeding problem...

A little background: my son was a preemie born at 35 weeks. He didn’t have the greatest sucking reflex when he was born, but that has greatly improved (he is now almost 2 months old). We have had no issues so far with him eating. Today, he ate well at the beginning of the day. He usually eats a full 4 oz no problem. This afternoon he did okay, but was a little slower than usual. This evening, it was a bit of a struggle to get him to eat 4 oz but he did. Tonight it took me almost half an hour to get him to eat 4 oz. he usually will eat 5-6 oz at midnight and sleep until 6 or so. My husband was able to get him to eat another ounce, but it seriously felt like we were back in NICU trying to get him to suck. Has anyone else ever had this happen?