Disappearing second line!?

Tara
Hi guys, 
So, still having all of these early pregnancy symptoms and AF was supposed to appear on the 5th after a really strong, confirmed ovulation with an opk on the 22nd. I am NEVER late on AF. 
Keep getting negative results so I waited 3 days this time to take another test. This past night I took a test at 2am because that would technically be fmu (but I woke up at least 3 times after between 2-10am to pee as well).. 
Anyway, watched the result window and I saw a faint second line pop up!!! Then maybe 30 seconds later it disappeared and only the control line was left.
Anyone have any ideas why this would happen?? Have a doc appointment for Tuesday but am so confused! 
Thanks everyone!