Body temperature. Of hot environment?

Hello, My baby has turned 5 months old on this 19th april. Since April 14th night he had mild temperature in between 99.2 to 99.8 f after 2 days I had consulted with his paediatrician where doctor has suggested to start the antibiotic cepodem 100 syrup, he had previous history of UTI due to which he was hospitalized at his early of 2 months old ,so just to make sure doctor advised us to do urine test for baby, for reference I have attached the test report. Since 20th he did not have fever . Now day before yesterday in the morning when I checked his temperature it was 99.4f and later it went to normal by itself on that day fever did not come neither on yesterday but today morning I observed his temperature rises again and below is the temperature note so far At 6 am arm 99.2 Rectum 100.5f At 9.30 am forehead 99.3f Arm 98.7f Rectum 99 .9 @10:30 am Underarm - 98.8 @ 10:30 am At 12.15pm 99.3f My question is why the fever is coming again? Is it because of hot environment?