So, first, there are right and wrong ways to take/read a pregnancy test. You have to follow the box instructions. You basically pee and insert stick. You can't have 'too much' urine when you do that so don't worry about too much in the sample. But a bit thing is to READ it at the appropriate time. All tests if you let them sit can develop an evaporation line that looks like a "yes" I'm pregnant line. But you aren't. That line just forms as the test sits. So, you read it typically about 10 minutes after taking it or whatever the box says and THAT is your answer. Not an hour later or something like that. Sounds like you saw an evaporation line.
This article may help you. https://www.webmd.com/baby/guide/pregnancy-tests#1.
Now, taking plan B can throw your period and cycles WAY off. It's a big dose of hormones and it can make you irregular for 2 to 3 months after taking it.
Maybe considering some form of birth control would be a good idea for you and your partner and then you will also not have to stress over possible pregnancy.