An axillary (under the armpit) temperature should be slightly lower than one taken with a tympanic (in the ear) thermometer. The axillary temp is also difficult to take accurately, particularly in a miserable, squirming bub! A tympanic thermometer is a good investment, particularly with kids. It's quick, easy and accurate. The Braun brand is reliable and very popular.

Sounds like you little man may indeed have low grade fever. I would keep and eye on him, offer plenty of BF's, watch his nappies (which you are obviously doing) and take him to the GP if your Mummy Radar says he needs it. I hope he feels better soon!