It's been 43 days since my last period. No Positive test?

Hello!!! I had my period Nov 17th. And now December is here and I have yet to get my period? I've tested a lot and got all negatives! This is our first month trying and we ended up getting too busy in the beginning of the month to have sex....except for 1 day the 20th of December. Was our anniversary! But that was the time I was suppose to get my period??? But it never showed up! 
Is it possible to get pregnant closer to the end of your cycle?? 
If soo then when should I test? If I ovulated so late in the month??  
Could this just be an irregular period??