Yeah, that's lucid dreaming.

The basic definition of Lucid Dreaming, is to realise that you're dreaming inside a dream. There's a lot MORE that you CAN do, but that's essentially what it is at a basic level: awareness within a dream.

As for why you would start having Lucid Dreams out of the blue? Not a clue, but apparnatly, going by what other ppl on the forums say, it seems pretty normal for this to happen out of the blue. It could be any number of things, I suppose... change in diet, change in family situations, an event in your day-to-day life, reading something about LD's inadvertently... a whole host of reasons, and I'd just be guessing...

But yeah. Welcome!