How soon to start next clomid cycle?

I am at the end of my first cycle (13dpo) and saw some spotting last night and been testing and getting bfn so I’m thinking I’m out. But... it’s possible to spot a little if you’re pregnant right? And I’ve read a lot of posts from women who say they don’t get bfp until a week after a missed period. But if I’m supposed to start clomid on day 5... How can I be sure I’m not pregnant before starting the next cycle? Has anyone gone straight into another clomid cycle after a failed one?