fever after 4 mo shots..

Hi ladies, my LO got his 4months shots yesterday and I was giving Tylenol as soon as we got home before a fever hit and it was working then at night LO was sleeping a lot so he skipped the next Tylenol dose and he got a fever.. it was 100.8.. I gave Tylenol and it went down to 100.0 but now it's back to 100.8-100.9 it's been 4 hours already since the last dose of Tylenol..I just gave him Motrin instead because my pediatrician said I could alternate between Tylenol and Motrin.. when should I consider taking baby back to the Dr or hospital? I haven't checked his temp yet right now so I'm not sure if it's down yet but is the fever normal? When should I be worried? I asked my dr at the appointment she just said if it didn't go down to alternate between medicine but she didn't say when or if I had to take baby back..what else can I do to lower his fever? Is 100.8 considered high fever? My LO is 20 weeks and I'm a FTM..