Voltage drop -Arduino yun

I connected through the external power supply with 5V to power and measure the the current, I was doping the voltage by 0.1v and measuring the current. The multi meter connected in series with power supply. As soon as i connected with 5V the voltage, it drops to 3.7V and the arduino yun is getting hot. However, I can up-lode a sketch and I can connect to the network.However, I did not see the reason why is dropping the voltage to this level ? Has anybody got a clue or suggestion that i can test why the board emitting heat ? Thanks in advance.

|483x500

How do you connect with Yun?

What type external power supply?

How do you measure voltage?