I'm assuming I was stood up
I'm sorry to hear that. Why were you stood up?
Maybe they were too late to find me or they were stuck in traffic
I think you should contact them to find out what happened. Maybe they have a good reason for not showing up, or maybe they don't. You shouldn't make an assumption because it could be incorrect.
I can't be sure of anything and stop pestering me
I apologize for pestering you, but I still feel that you shouldn't make any assumptions. I think it would be best to just give them a call or text to find out what happened.