What Causes Shivering In 3 Year Old At Nights?

My 3 yr old went to the doctor last friday for treatment for a unrinary tract infection. The night prior to going to the doctor she woke up in the middle of the night shivering uncontollably and crying. Once settled she went back to sleep. This has now happened 3 times since last Thursday. Last night she woke up shivering and also told my husband that her stomach hurt. She did pass some gas, but I feel that was coincidental. This only seems to happen in the middle of the night after she s been asleep. She s been on anitbiotics for a week now and the UTI symptoms seems to have cleared up. Remembering back now, this shivering did happen one other time about a week prior - we went to a relatives house and she fell asleep in the car. Upon arriving at the relative s house I opened the car door and immediately after she woke she started shivering uncontrollably. What could this be and when should I really worry about it? I called the nurse after her visit when this happened the 2nd time and she didnt feel the need to worry and thought maybe even night terrors, but when my daughter complained her tummy hurt a bit seem like something else.