Tracks keep changing pitch while playing.

I'm a club DJ. Have been using the same USB with my tracks analyzed through rekordbox for months. Recently got a new laptop, so had to install rekordbox all over again and load my tracks into it again. Everything was going smoothly until I rocked up to my club gig on Friday and noticed every track I played was changing pitch mid play without even touching it. I first thought it may be the old club equipment, so when I got home I played on my own system and the same thing was happening. I've never experienced this before while DJing, but my only thought is it has to be either rekordbox or my usb.