98GTstang

I just got done rebuilding my t-45 yesterday got it in last night and its been perfect all gears are good and the shifts are smooth..but tonight i was at sonic and when i went to go into reverse it went in fine but then tried to go into 1st but it was jammed in reverse. i used reverse over 10 times since the rebuild and all of a sudden it locked up..after about 20 minutes of messing with it it popped out.i used mobil 1 synthetic atf fluid and i put in about 3 1/4 quarts..do any of you know why this happened?