What Causes Delayed Periods Along With Dizziness And Nausea?

I am 26 years old and I came off the pill 3 months ago to try for a baby. I have been having unprotected sex since then. I had been on the pill for approx 10 years. I had withdrawl bleeding when initially coming off pill then my period started 27 days after this. I then went 36 days until my next period started. I am now 37 days this month and still no period. I have done several pregnancy tests with negative results. For the last few days I have felt nausea and dizzy which has been around tea time and it's only lasted approx 2hours. Have you any idea what this could be? Could I be pregnant? Thanks.