Monitor Conflict

Hello,
I have 2 monitors fertility.
Use one in the morning and another in the evening.
Last month marked the two the same thing and not in conflict.
But this month the monitor in the morning is marked high fertility and low fertility monitor at night, I'm not going to the bathroom for 4 hours to take the test the night.
Two days ago the monitor brand high fertility in the morning and the evening low fertility monitor. This morning marked the high fertility monitor again!
Why is this happening?
Thank you
Sorry for bad english, im brazilian and using google translator