99. 4 fever. What can be the issue.

Hii.. My sister who is 29 years old is having temperature variation through out the day since few days...(since we started monitoring before that we have never monitored) When she wakes up temperature is 98.4-98.6 But as the day progresses her body temperature rises to 98.9-99.4 Before sleeping her body temperature is again normal 98.6-98.8. There is also weather change in delhi, the temperature outside has been too hot. Secondly she doesn't have any symptoms no cough or pain or anything else. She has doesn't even feel that she has fever or something, is completely active, eating proper food, proper fluid intake and sleep. Blood and urine tests were done last month and everything was normal. We have never monitored the temperature this is the first time that we are monitoring it and found out the issue Please suggest what can be the issue...