this has been happening to me every few weeks for a few months. i'll feel very feverish, but then take my temperature and find that it's below normal (usually between 97.6 and 98.2). i've tried to notice a pattern, and it seems to happen most often when i've been very busy and stressed, accompanying exhaustion from work, life, anxiety, etc. also, my environmental allergies have been AWFUL this year (most allergy sufferers have had a terrible year for allergies this year)...i've had congested sinuses and a sore throat off and on since march, so i think it may have something to do with that as well.
it may also help you to know that it's completely normal for your temperature to vary by as much as 1 degree throughout the day. and a "normal" temperature of 98.6 is only really an average. your personal normal may be a bit lower than 98.6, which is completely okay. for example, i know my personal normal is 97.9. so if i vary by half a degree and have a temperature of 98.4, i feel quite warm, although it's obviously not a fever.