@martial ! First of all, learn to NEVER apply a single brake, whether its front or rear.
Second, kindly head to nearest Wheel Balancing Shop, and get the rear wheel checked. It could be that:
Rear wheel's fitting is out of alignment ~ In which case, you can do it yourself easily. (Check the rear Tyre's Axel, there will be a few Engraved Bars on the either side of the Arms holding the tyre. Make sure the bars' adjustment match on both sides)
Rear Wheel itself is out of alignment ~ In which case you'll have to ask a Wheel Balancer to correct it.
Its the most common issue in new bikes, the rear tyre itself is faulty i.e. its hardened hence no grip on the road ~ In which case you'll have to get a Brand New tire.
Also the most common issue, the rear tyre's alignment (i said "TYRE's Alignment") is faulty ~ In which case you'll have to change your tyre.
ALWAYS apply BOTH brakes Simultaneously, with 70-80% of the pressure on the Front, and only 20-30% on the rear.
While braking, Clutch your Knees with your Fuel Tank, for Extra Grip.
Thats THE KEY on braking CG125, with STUPID Drum Brakes.