It can be due to hot environments.... Keed hydration normal by frequent feeding

Temperature less than 99 is normal for the baby. Environmental heat can be a reason for a slightly higher body temperature of the baby.

If the child is breastfed,if the mother has no illness, and most importantly if the child is playful and taking normal feeds, just don't bother

Baby temperature  of infants till 1 year  is slightly  higher than the normal temperature.. This is due to the  growth process  (metabolic  activity) is higher in  this  age. Always  look for the charge in your  babys activity,   If the  babys activity is  less  during  high  temperature  record, then think of  fever. Give  vit D 3 supplement  and after 6 month  calcium  supplements  till 1 year .

Hi 99F is not fever....if he feels feverish keep on monitoring and keep an eye on baby's overall activity.....if temp > 100 or reduced activity then you need to consult It can occur in humid weather conditions