Transmission Issues.

I was testing out top speed in my 4.0 2007 v6. I was on a country road by my house, nobody ever drives it so its perfect. My car is an automatic (3 gears and a 4th overdrive gear) I was nearing 100mph I think it was either 3rd or after my OD gear and my car tried to kick down to 1st for some reason. It made a really loud revving sound that scared the jeebies out of me. Could anyone give me some input on whats going on here? I feel like overdrive just didn't happen or it was after it. mods: catback xpipe and true duals pypes performance exhaust, bbk cai, diablo sport tune (93 oct) Please help :)