lombii, I googled Ameixa vermelha and the pictures sure look like what we in the USA call Plum. A Prune is a type of plum with a high sugar content that dries quite well.
Plums and prunes are the fruit of a plant in the Prunus genus. There are many varieties, but the leaves of all are quite similar. The leaves are wider than in your picture and have toothed edges. They are not so smooth, either. The veins are usually more distinct.
They are quite willing to grow from seeds that drop from trees, or that birds carry away, so it is possible that a fruit you ate sprouted. I sure doubt that the plant you have posted the picture of is the result, however.
I agree that the leaves do look a lot like Oleander, but somehow, not. Oleander (Nerium oleander) leaves grow in a whorl, often 3 leaves around a somewhat angular stem.
When i was removing the plant from my inside garden, i found the its seed shell - at least one side of it (pictures uploaded),
and then my wife saw it and identify as a plant called AGUAÍ (Chrysophyllum lucumifolium), a little tree as you see in all itīs shapes
on the link: