When should I take a pregnancy test if I have only just come off the pill?

Patient: I have been taking cerazette for 18 months now but two weeks ago I ran out and have only just had time to get more. I have to wait until my period comes to take them as I have had unprotected sex with my partner a few times. For the past few days I have been feeling very tired, having lower back ache, period pain like cramps on and off, nausea, sore breasts and have been feeling emotional. I took a pregnancy test two days ago and it was negative but as I dont know when my period is due could it be wrong?