What Causes Delayed Periods While On Chlomid?

I have been seeing an obgyn to try to get pregnant. I was having difficulty getting my period. So he prescribed me progesterone in june that I took for seven days. I got my period 4 day s later and Then I started chlomid on the fifth day of my period. After that I have been getting my period all by myself without any medicine like clockwork. I took chlomid two more times in july and October and we have been having sex frequently ever since. I finally took a hsg test on November 4 and I got my period December 1st. I have not seen my period since. I took a test January 6 but it was negative. What should I do.