I feel near to sure this is Begonia hirsuta, the only problem is that according to the 'Flora do Brasil' the plant doesn't occur in the Rio de Janeiro State, but in the immidiate neighbouring state: Espirito Santo. As I found the plant in a touristic preserved part of the Atlantic Forest (Rio de Janeiro) with daily visitors mainly Brazilians from different parts of the country, I find it very possible the dust like seeds have travelled...
The 'Flora do Brasil' also sees now Begonia hirsuta as a synonym of Begonia humilis var. humilis, but the Plantlist not..
http://floradobrasil.jbrj.gov....
http://www.theplantlist.org/tp...
I recognize and based my identification of my plant on this;
https://commons.wikimedia.org/...
they got their material from the National Botanical Garden of Belgium..

I know it also looks similar to Begonia hirtella but I've seen both in person on different occasions and there are slight differences like the flowers of B.hirtella are bigger, the ones on ‘my’ plant are very tiny and another distinguishing feature is that the flower stems on my supposed B. hirsuta are always red and the base of the leaves with a red dot and more undulated between the veins..
The first photos are in the location where I found them:

The next lot are from plants I've grown from the seed I collected at the time, (don’t ask me why, the plant is not showy at all, but it had seedpods so couldn’t resist! )

I can see why you would want to grow it Myriam, the leaves and tiny flowers are attractive!

Begonia hirtella is the one I found before I read your last part about it, flower size might differ according to age of the plant?

There's several photos of B. hirtella to compare on phytoimages ..

http://www.phytoimages.siu.edu...

Delta-intkey has an illustration which shows a red dot at the leaf joint, and reddish upper stems.

http://delta-intkey.com/angio/...

Begonia hirsuta as shown on wiki has very hairy flower bracts which yours doesn't have.

Thanks Janet, it is only not very clear to me where the begonia flower bracts are..they should normally being attached to the actual flower petals?
some more close-ups, handy when you have the plant at hand!

Now I'm not sure what I thought was flower bracts are bracts, they are below the flowers on the photo on wiki, two quite hairy protrusions but looking again they might be new leaves emerging? If you keep clicking on the photo it keeps getting bigger.

They are very similar! Several drawings on plantillustrations of B. hirsuta show serrated leaf margins .. yours doesn't appear to be very serrated, although they look to be slightly serrated which might be normal.

http://www.plantillustrations....

Begonia hirtella .. the leaf shape is more rounded like yours.

http://plantgenera.org/illustr...

The botanical drawings are more clear than the wiki photo, Janet. I'm still recognizing my plant in there, but maybe it is wrongly identified..
I'll keep it as Begonia hirtella.

Here another picture of Begonia hirsuta on another Wikipage, it looks so much like mine!
https://pt.wikipedia.org/wiki/...

OK, I think we need to look more closely at the flower seed pods. On each flowering stem there appears to be three flowers, male are the simple ones with the female flowers having inflated pods and smaller petals below the pods.

I've looked closely at the drawings, those of Begonia hirtella look a good match. Note how there is one 'wing' a little larger than the others on the seed pod, but the larger wing isn't much bigger.

http://plantgenera.org/illustr...

Now, look at the seed pods on drawings of Begonia hirsuta, one wing is much longer and narrower than the others, which don't look to be very big at all and look to be barely existing.

http://www.plantillustrations....

very true Janet, the drawing of Begonia hirtella is a perfect match with mine, I am only confused that the photos on Wiki of Begonia hirsuta don't seem to match the drawing of Begonia hirsuta, in fact looks very different..
I went straight to the National Botanic Garden of Belgium, see the photos don't match the botanical drawing, but do look like my plant..
http://www.botanicgarden.be/RE...

Good observation of the difference in the seedpods of both!

Made a close-up of the seedpod of mine, clearly not a match with B.hirsuta from the botanical drawing.
I can only conclude that the plant at the botanical garden is wrongly identified!