Rexes have beautiful (seasonal) flowers, tho' of course they're grown more for their leaves. Serratipetala is not a rex. I only had it briefly some years ago and don't remember much about it, but I don't think it's unusual for it to bloom either. You might check the website of the American Begonia Society for more information. Blooms on any begonia are an indication of good culture, so congratulations!

Greenelbows is correct. Rexes bloom as well as any other begonia (although there are some begonias that have been reported as never bloomed). Most rexes bloom late summer, early fall (most of mine are blooming now) and usually have blooms that range from pink to red on the outside with white on the inside.

Serratipetala is a shrub species from New Guinea and can grow quite nice if given plenty of humidity (mine is like a weed in an old aquarium). The growers that provide these rexes to the big 3 box stores usually label (in this case mis-label) all of these begonias as Rex Serratipetala which can be very aggravating. I think they must've had a million of those tags printed and won't change them anytime soon.

First off, thanks Greenelbows for the congrats.

and hcmcdole, i believe you are correct with the mis-labeling of the rex, the only thing i am certain of is that it's a rex.
And i have this great little unexpected flower.

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i wish my camera could capture how the center (pollen perhaps) looks like four long curly yellow strands, not just a blob of pollen.
also, the flower opens during the day and begins to close as it gets colder, until it's practically closed, just to spring back open when it gets warm / light.

i may sound like a complete novice to gardening, because i am. and this is my first houseplant. But putting it in my very small bathroom, with a frosted west facing window seems to work.

thanks for the input , and i hope the pics help.

Here is a link that might be useful: pics of my rex