late period - help a girl out pls

So I got my last period October 25th to November 1st... eve said my next period was dude dec 1 but it still hasn't come it's now the 23rd (23 days late) I had unprotected sex like over a month ago. So a week after my last period ended. I took a test EXACTLY 2 weeks after it was late, came back negative and I took one today. And it came back negative. If I'm not pregnant what's causing it to be late??? AND I'm getting jadelle rods on the 26th of Jan, is there anything I should know? Like do they hurt and is there any side affects?? Thanks for any help! :) 

(Both tests were done at a health clinic and youth services)