Flat BBT?

Ems

Anybody can you please give me an insight on this? I’ve been temping for months and this is the 1st time I had this kind of chart. My temps are pretty straight forward on previous cycles... it always go down right before it comes up very high during O and it stays there. It was never like this. I checked my thermometer and it’s working great, battery was changed last cycle. Thanks in advance!