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.