Tranny problems?

I was wondering if anyone else had experienced this and what the problem was. I was coming up to a stoplight and was going around 35. I braked, not hard at all, and when it was going around 30, the transmission downshifted (to 2nd i think)(I have an 2000 with an automatic.), but it was very jerky. i have felt it shifting before in similar situations, but it has always been relatively smooth. This acted like i was going 50 and manually slammed it into 2nd. Has anyone experienced this? Is there anything wrong or am i being paranoid?

make sure your gear selector was in "D" and not "3". What happens is that when you have it in "3", it will downshift at 30 MPH to give you compression braking in 2nd gear.

Yeah, I know that if it is in 3, it will shift down @ 30, but this happens when it is in drive.