My gluco level goes up without eating?

[ONE]

I have diabetic (2nd one) i did ate 3 slices of pizza yesterday at 3:00 pm, i took 1 tablet of Glucophage (metformin) 850 mg, in the evening i have gluco. test level it was 98, after that i did not eat any thing till the next day in the morning, in the morning (after 18 hours) and before i have my breakfast i have new gluco. test the results was 148... may some one explain how it going up without eating? i only drank water n coffee with special sweetner?????