@chloematthews99,
You don't have a lot of choices. You can confess to your boyfriend now or you can confess to him sometime in the future if you two are having a confessional conversation or if your ex spills the beans. Or you can do nothing.
Since you have no contact with the ex and he's no longer a part of your life my guess is that this concern will fade over time. What possible reason would your ex have for finding out who your boyfriend is, obtaining contact information, and informing him that, "Your girlfriend sexted me." It doesn't seem likely.
A nagging conscience shows that you have ethical guidelines — that's a good thing. But if you don't want to be driven crazy with guilt over things in the future you need to reflect more carefully on the decisions you make. Listen to that voice in your head, but earlier on,
before you make a questionable choice.