Were your hands clean? Did you recheck? Starting higher than 235 can lead to increases in glucose levels, indeed, in the UK, it is not recommended to commence exercise with bloods above the 235 level.
Alternatively, although after five minutes a little unlikely, it could have been a liver dump. If you were doing any sort of anerobic exercise it uses the glycogen in your muscles to provide energy. When this is depleted, the liver converts its stores to glucose, dumps them into the blood and your muscles extract it to provide energy. Now I haven't seen it after five minutes, but it is possible.