mirena IUD, possible pregnancy

Lu
Bit concerned about the number of people getting pregnant Even though they are on the Mirena IUD... 
I've been in it since March this year and normally always have a period but it's not come this month but I have also never been able to 'feel my strings' so getting a bit concerned.. not feeling like myself either constantly feeling sick, tired, peeing for Britain! Headache, sore breasts, feeling faint... so going to buy a test tomorrow but was wandering has anybody on this page got pregnant whilst being on the Mirena ? Thanks!