Period delay. Has happened to me?

Hello, I'm 25 years old female. I got menstruation on 12th Novemeber 2018. I started hardcore gymming from October. I never had delay in periods before this. This is the first time its happening. I visited doctor Pallavi Nikam on 15th december and told her everything. She asked me to wait till 19th dec. I didn't get my periods. I did USG Pelvis too. And showed it to my doc. Doc asked me to do pregnancy test as well although I am not sexually active. Still she wanted to rule out. So I agreed and did. The result was negative. I was prescribed to take primolut N of trength 5mg 3 pills everyday for 5 days. But still haven't got my period. Can you please tell me what exactly has happened to me? In my USG pelvis report The right side ovary has been mentioned as little bulky and a presence of corpus luteum cyst has also been suspected measuring 17mm.