Question for women with knowlege on ovulation please!?

my boyfriend n I have decided to try for a baby- this is our 1st month of trying and therefore my 1st month been off the pill- because i dont no the lenghth that my cycle will be becuase ive only jst come off the pill and my bodys stil gettin used to been bk in control i ***** use ovulatiuon calenders so i invested in a digital ovulation tester- if my cycle is 2 b 28days i was supposed to ovulate last week so ive been testing since then- everyday had said no- until this morning when it sed i was.. however my boyfriend is working late tonight n i let paranoier get the better of me n did another test a few minutes ago n this tym it sed no. How can it change in the matter of 12hours when it said no yesterday aswel- does this mean ive missed my chance? Please explain...thanks in advance.. (also im from the UK- just to explain the tym difference)