Yes it can be positive for just one day, most are only positive for one day. I had a positive for only one evening and it was gone the next day.
It is best to test 2x a day, usually between 11-noon and 8-9pm. Ya I def. could not use the clearblue, they are to expensive. I use the Answer brand dip sticks, it comes with a pack of two little sticks you dip.
Here are some pictures...
This is a positive, even though the WHole line is not completely darker then the control line, the outside line on the test line is Darker then the control line , so that makes it positive. I wanted to make sure so the the test below it is a Dollar tree test I took to back it up.
Dollar Tree test I used to back it up a few mins later
Here is a picture of all of them together counting up to the day I ovulated. I ovulated on Cd 17. Some ppl's tests gradually get darker, and some just turn positive out of now where.
Do you have a picture of your test?