4 month vaccine, day 5 fever

Hello! I'm wondering if anyone here has gone through this. My 4 month + 2 weeks (3 months + 1 week corrected age) got his 4 month vaccine 5 days ago. The first night his fever was 38.6 then 20ish min later 39.3...we gave him Tylenol as directed by the pharmacist. It did go down a bit but still stayed a low grade fever. We gave him Tylenol on and off until Sunday. Maybe 4 or 5 doses in those few days. He started acting himself again yesterday and smiling and all his energy and more so again today. But the fever keeps coming back. It's on/off. It seems to start back around noonish. His pediatrician is on vacation and emergency appointment is always booked even when you call at the opening hours. I don't want to go to ther ER since the last 2 days it doesn't go above 38.3 but I'm scared and not sure how normal this is. He was born at 34 weeks, IUGR, even a hernia surgery at 6 weeks old so I can't help but worry. He eats well and otherwise I wouldn't even realize he had the fever if I wasn't checking.