Pregnancy Testing?

Kendal
I had sex on the 10th of this month. Had a "1 period" on the 11th and so far nothing on the 12-13th. (We have had sex prior to this I can't remember when though) is there a possibility I am pregnant? If so when should I take a test? My period is never regular and I sometime go months without one so I never know when it is late. 
TIA!