Not pregnant, but where's my period?

Tory • I'm in love with my best friend and I couldn't be more happy.
So I'm a very regular period haver, every month around the 21st/ 25th I'll have my period. Last month I didn't have a period. I started to get worried as I have been having protected sex (with condoms). But the test came back negative where's my period? );
 P.s- I struggle with ovarian cysts and chronic depression. And I did have a fair amount of stressors in December like almost being evicted for "not paying rent"(banks fault). 
has anyone ever had a regular period and it skipped a month? I've had my period since I was 9. I'm 19 now. I'm also slightly confused.