13dpo/BFP?!!

Okay so I took a cheap test this AM. I'm 13DPO, and I waited 3 minutes and it was so very negative I threw it in the trash. 1 hour later I had to pee so I did and when I glanced in the trash I saw a very faint line. I know MOST tests say don't read results after 5 minutes because they can be invalid, but now I'm confused. Should I buy an expensive test tomorrow and retest with FMU?! Help. Ahhh! I also just had endometriosis surgery so my period is already late by 1 week. But I happened to take my opk's still and had my positive 13 days ago. What would you do/think?