Hmmm...
Technically speaking, it could be few things.
Still reaction to the removal of the coil. I had this issue after coming from the pill, I spotted one month.
it could be that it is reaction of the body to potential ovulation, but not sure how fast the results will show. As the test shows whether you ovulated.
And it can also be that it is your body reaction to non ovulating circle. From what I heard, it is normal for 1-2 circles in a year to be non ovulating. So it may come to that that one month you will not be able to find any trace of ovulation, because there will be none.
It can also be, that the test is showing false results, but I dont have experience with these tests, so I cannot say.