Positive solid smiley and blank circle few hours layer

Hello,

it’s my first month testing so a little confused. I tested on Friday afternoon and it was a blank circle, I then tested on Monday afternoon and got a solid smiley ( I was travelling so forgot to test in the middle). Just to be sure I tested again on Monday night and it was a blank circle. Does that mean I am not ovulating anymore? Or since I didn’t test on Saturday and Sunday I missed ovulation? But how come the smiley face appeared on monsay afternoon but not at night? 

thanks