What Causes Delay In Periods?

I've been on the pill for 3 years but in feburary I change from Yasmin to Orthotricycllyn low. My last period was march 11 and since I saw that even on my sugar week pills I didn't get my period I didn't continue the pills. On abril 12 I had a blood pregnancy test done, the results were negative. I haven't gotten my period yet,and have done 2 more urine pregnancy test done both were negative but I have all the pregnant syntoms.What can be happening?