Looks like that's emersed growth, judging by the flower and the thick stems. Wait a few more weeks until the submersed growth appears, then we'll be able to tell.
I had a similar thing happen. I bought a plant labelled Mayaca, and I was so sure it wasn't, because it has small round leaves. Well, a week underwater and the new leaves were long and skinny - absolutely Mayaca.
I've never seen emersed Bacopa, but your leaves kinda look like it. Maybe even Micranthemum, but I'm not even sure if it grows emersed. Or it could be something totally different, with different leaves when submersed. Give it some time. Don't you love the suspense?