Its like a grinding sound when i press on brakes

Nico, return to the mechanic and tell them of your problem if rear left is still grinding or right, have them repair and make it right.
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Fixing Grinding Brakes
Rear brake grinding
What Making That Grinding Sound When You Hit the Brakes
Solving Brake Problems

Sounds like something came apart. Go immediately back to who ever worked on them for an inspection before major damage. If you do not like the answer (you just wore them out and it will cost......) Go to someone else now. Grinding noises from brakes are never good.

If You are hearing a grinding sound when you press on your brakes it probably has something to do with the brakes again so I would take it back to the place that replaced them 4mo ago and have them fix it cause they should be responsible for your brakes if you just paid them to replace them 4mo ago

Hi,
If it is a grinding noise then you have a problem with the brake pad area, it may have a stone caught in this area. Or it may have worn front brake pads.

The brake pads sound worn and are grinding into the rotors due to low lining on them.Have the brake pads replaced, hopefully they did not ruin your rotors with grooves yet.Please let me know if this helps

Check caliper for correct operation. The sliders, did you grease them?

The grinding noise in the rear brake drum means that brake shoes are warn out and the metal part of the shoe is touching the drum. You need to replace it asap as it will damage the inner part of the drum and than replacing shoes will not help without skimming or replacing the drum.

sounds like you might need new brake pads

Sounds like the pads are worn out and are grinding on the rotors. You will likely have to replace the pads and rotors.

I would look for rust on the back side of the rotors; this is a very common problem that will duplicate the noises that you describe.