I just got got my transmission rebuilt and it was working fine but now the first gear takes off really slow so I have to start off on the second gear. I was told that it could be something electrical.

 

Need more info... does it have manual first gear? Are you in OD when this happens and you down shift to ??? You should have a 7 quadrant gear selector. P R N OD D (3) 2 1. It sounds like it is electrical and in Limp mode or fail safe. Takes off in 3rd should be able to downshift to Manual 2nd. Take it back to the trans guy for a scan. Maybe the main connector or something got disconnected and has it in limp in.
Tranzman