but now my tranny is slipping

I tried to change the oil in my truck when apparently i changed the oil in it when it was all said and doen i turned on the car and put it in drive.

So I left it at that ...well I got someone over here to fix the problem but now my tranny is slipping and was driving it for 20 min it stopped but I just don't know if I created a problem can u help me please or give me advise on what I should do. Thank u