Question on ovulation test

Ok so I have been taking my ovulation test daily and I am aware that once you get a positive u no longer need to test. On Tuesday I got my positive then on Wednesday I got a negative and today I just took a test just because and although I got a negative I did notice the line did get darker than yesterday. Should I continue testing or is that normal and will the lines just vary throughout the month based on how much of that hormone is in your body? I just so curious as to why the line would get darker