The only explanation I can think of is that the liver releases glucose to feed muscles when exercising, this may of tipped you over, unless you have an infection of some kind going on which would cause you to go higher than the level you started out at.
could be aswell i last ate about 12 hours before previously last ate about before 10pm at night then went out in morning walk empty stomach so thats around 12hours after last ate
When did you last do a basal test? Another possibility is that you rose to 12.4 because basal needs adjusting. There are a number of factors that could have caused the rise such as illness, stress, insufficient basal, or as @Juicyj suggests a release from the liver to feed muscles. Try to look at each to see if they fit.
Muscles need glucose, so your liver releases it. But if you haven't got the insulin to get the glucose into the muscles, it's got nowhere to go and just hangs about in your blood.