Hmmm, ok, well, I'm not sure.
I have in mind that she might have gone through the 18 month regression earlier than most and that this disturbance at 20 months could be a language leap usually seen around 21 months but I would have expected NWs along with the BT reluctance and nap disturbance, as she's still STTN maybe it isn't this language leap at all.
So, perhaps teeth? Anything on the move? 2nd year molars perhaps?
Could be a blip, cold coming on perhaps. Has she had jabs recently? (I forget when they get them without looking it up)
Some times when it's not obvious it's worth just waiting and seeing what happens next.
I'd be tempted to leave her routine as it is and see if she comes through this in a week or so. The other option I can think of is to cap the nap and see if her night lengthens, and then see if she is in a better mood with the longer night, but it's not going to happen immediately, after all you've had some short naps and didn't mention she was happy so I assume she wasn't.
Sorry I don't have a clear answer for you.