This is gonna be long but I need answers.

So, yesterday my daughter was with her grandma. When her dad went to pick her up he told me she was really warm. I told him to wait a few mins because she had been under covers. She was still warm about 10 min later so I had Gma check her temp. Under the arm was 101.8 and rectally was 98.6. Since it had went down I was relieved. Well when I got home she was on fire! And grunting like she was uncomfortable. I didn't even bother checking her temp I took her straight to the hospital. Her temp was 103.6! I balled my eyes out. We were there for about 4 hrs. They did every test possible. X-rays, blood tests, swabbed her nose, checked her ears and even tried a catheter to make sure it wasn't a UTI. EVERYTHING came back negative! So the doctor said it's an unknown origin. Has this happened to anyone before?? I feel like there has to be a reason why she had a fever, it makes no sense. Today she is fine, no fever and acting like her cute little self. I just don't get it. Anyone experience this with their little one?