Well, a lot of times things are only coincidence, but we try to connect them to certain events that happened, just so we can freak ourselves out
Consider, though, that canker sores are an autoimmune response to something that has happened in our mouths, like biting our tongues, scratching our gums with food, etc. Perhaps the oral sex you performed compromised the integrity of your oral mucosa in some way, and you got a canker sore.