Naptime suddenly got more difficult!!

So, Alex has pretty much been the type of baby who enjoys sleep. He gets that from his mommy. When it's nap time we rock, sing, drink a bottle then as long as he has his little sleep cue blue bear we call "teddy" he's off to sleepy land as soon as he's down. Today however, we entered a whole new world. We went through our routine and I put him down for his afternoon nap. I went about cleaning up the house like normal. As long as I don't hear anything I know he's asleep. Well, I heard him baby talking his "da dah dah ala dah." I went in and found him like this. He had pulled himself up to his knees and was having a ball not sleeping. Earlier today I found him sitting up in his crib, but didn't get the camera in time to record it. I'm not sure if he pulled himself up with the bumper, or if he's learned how to sit up by himself. I'm afraid of what's to come. Suddenly naptime has become more difficult!
Funny that Duane and I had been agreeing it was time to lower the mattress on his crib, but just couldn't get to it. Well, Daddy did it this weekend and it's a good thing he did!