Dye run?

I took a pregnancy test this morning as I have had the exact symptoms I had on my daughter. Main one being spotting after sex last week.

At first glance it looked negative. I put the test next to me on my bed to let it develop.

My boyfriend came in the room and said we’re having another baby then.. I was like no?? So I looked at the test and it was blazing positive! But within our conversation about it we looked at the test again and the positive line slowly moved across the window. Then disappeared.

This all happened within the time frame however I’m guessing it was a dye run?

I’ve never known of this before and I can’t find anything about it on google as a lot of things say a dye run stays as a smudge and is visible, nothing says anything about moving across the window.

Most likely negative but has anyone experienced this please?

TIA