A Grown-Up's Playtime?

I'm in a long-term relationship where my partner looks to me for guidance and discipline. I've never called myself 'Daddy' or anything like that, just kind of slipped into the role over time. We have fun with it, timeouts, playful scolding when she gets out of line, maybe a little spanking if things get really heated between us. But tonight she asked to be treated more seriously than usual...and I'm not sure where this is going or if it's normal for us. Anyone else ever experience something similar? Am I crossing some kind of weird line?