umm i think I got the problem what user is having.
@ssam! Brother you said its a new bike, and it only tend to skid at lower speeds while not higher. (You also stated that you use brakes in Pump Style which means you are a experienced rider not a starter)
There could be TWO possible reasons i can think of.
Tighten your Rear Brake a little bit. I experienced it myself that loose brake pedal works very good at higher speeds but bake shoes (or whatever you call 'em, drums whatever) does not Grip a trye (when you apply brakes) at lower speeds, and when you press it a little harder, the rear wheel jams and the bike tens to skids. (Be assured i am talking strictly about CG-125 Classic/PowerPlus model and the REAR Tyre of it)
Rear tyre alignment is out of order. Get it checked.
However i still think reason 1 is your problem.
Please check and reply back.