This just happened to DD! I am amazed by it as her mama, but it didn't surprise me b/c this is how she hit every milestone. Nothing, nothing and then bam! Everything! LOL
My mom remarked one time...I think when DD started crawling, that it's like a lightswitch gets turned on in their brain. Nothing you can do beforehand and nothing afterwards.

I thought DD was strange in the verbal dept. She could say complicated words or express odd conversational bits, but then she couldn't say yes or no, or ask for a particular food, etc. She would point and grunt, then turn to DH and talk about a video game. It really really worried me lol.
Case in point...she went through a phase where she refused to call animals by name. A cat was a meow, a horse was a neigh-neigh, a mouse was a "ee ee ee!"
Now she won't stop jibbering. DH turned to me this morning at 5am and said, "Can we put her back in her box and return her to the store?" I retorted, "Sorry, hun, I already recycled the packaging." (Pregnant)
