Mystery cactus

A few years ago--maybe five or six--I found a cactus leaf on the floor of a local greenhouse, next to a floor drain. A single leaf, rather small and wan, already limp from having been knocked off its mother plant--but looking around, I couldn't locate the source. I slipped the leaf in my shirt pocket and, fortunately, remembered it at the end of the day. After floating in a cup of water for a day or two, it seemed visibly revived, and I had not much trouble rooting it. I think I had to wait almost six months before it actually put out a second leaf.

Fast forward to last year: the wee cactus has been steadily growing in a narrow wall pocket (planter) in my bathroom and has four or five arching stems. Then, finally, it popped: seven or eight tiny flower buds that opened nearly all at once, after growing and growing for a couple weeks, into gorgeous shaggy multipetaled crimson blossoms. Such a nice surprise.

That was last year. This year, the cactus is a bit larger, though still on the smallish side, and still growing in the same planter. There's finally enough of it that I felt comfortable removing a three-leaf section to root for a friend. And it's blooming again, only this time one bud will appear, grow and swell for a couple weeks, and finally open--expanding open during the day and half-closing at night. Each blossom lasts about a week before closing up for good and eventually dropping off. Meanwhile, a second bud starts to develop on another stem, and then another, each taking its turn. It's a nice extension of the blooming period, but I don't know why it's behaving this way: the plant has been in the same location for years now.

Oh, and I don't know its name. Do you?