If you suspect your LO is sick how frequently do you take his/her temperature?

I suspect Sadie might be getting whatever sickness I have because she's been fussy & won't let me put her down, which isn't like her. I've been holding her all day. Her rectal temp was 99.1, which is higher than normal, but not cause for concern. I know I shouldn't keep putting something in her bum, so I don't know how frequently I should check. I don't want it to get high enough to warrant a trip to the ER.