weight painting one side of my model affects the other side in a strange way When I'm weight painting at the right side, it acts strange on the left side. What am i doing wrong?
You are using x-mirror option:



But, if you name bones correctly,  x-mirror will work for you. You need to name pairs of bone with postfixes ".R" and ".L" like "arm.L" and "arm.R". If you do so, when you will paint weight group "arm.L", "arm.R" will be symmetrical to "arm.L"

Also, learn about rigify