What is going on???

Ok, so here is an odd one - I tested my sugars last night before bed and they were 4.2, which for me is a little low for sleeping, so i ate 2 digestive biscuits before sleeping. Then I woke up at 1.40am and just out of habit i always test my sugars, they were 5.5. No problems there. Then I woke up this morning at 6.00am and tested and they were 9.8 (a bit high but as it was odd I thought I would leave it until 8am when I eat breakfast to see what happens)...I then tested my sugars at 8am before eating and I was 17.6!!!!!!! What on earth has gone on here.....all I had had in between was half a cup of tea (no sugar, tiny bit of milk).....please can someone explain?
I dont feel unwell so surely it cant be due to illness.....so bizarre