Why CAN bus impedence changes when I send data?
The mistake is the way of measuring the impedance. A multimeter will output a precise current to the load and measure the voltage drop, U = R*I gives the Impedance R.

If there is already some data traffic on the bus wire this means also additional voltages on the wire. The multimeter can not distinguish between voltage drop due to the injected current and due to data transfer, so leading to faulty impedance readings on the mulitmeter.