I didn't think anyone took their temperature rectally anymore??? except for babies and animals.
Was your sis in law pulling your leg?

My personal theory is - a fever or temperature is just a number and can fluctuate thru the day.
If you feel sick that is the more important thing. {this is just my take on it}

But,
if someone has a very high fever {between 103 and 106 F}, especially with an infant, elderly or if there is other health reasons to cause concern... then it's something to keep an eye on for sure.

It could have been the food or anything could have caused a slight temp increase like that, as long as things get back to normal and you don't feel sick, you should be fine.

We use a laser temp thing that is actually a mechanics tool. http://www.amazon.com/Raytek-MiniTem...=pd_sbs_auto_1
we used a digital thermometer when the kids were young. I can usually tell with my hand if someone has a serious fever or not.

temp/body orifice info
http://www.nlm.nih.gov/medlineplus/e...ages/19700.htm
fever info-
http://www.medicinenet.com/aches_pain_fever/article.htm