Arduino UNO - analogRead() returning mixed results
The analogWrite function, despite its name, outputs PWM (pulse width modulation) which is either fully on or fully off, with varying duty cycles. If you run that through a RC network (eg. 4.7k in series and 10 µF to ground), it will smooth out the pulses and you will actually get a varying voltage.

So in your case you are measuring on or off depending on the timing. However since you are measuring between the resistor and the LED, you won't get 5V because of the voltage drop over the resistor. An LED, depending on the colour, might have a voltage drop of around 2.2V, and thus you might expect 2.8V at that junction. This is approximately what you got.




  I want to measure the Voltage of the current ...


This doesn't make any sense electrically. You are either measuring the voltage (in volts) or the current (in amps). You don't measure the "voltage of the current". I suppose you mean "the voltage at that point".



Reference

Arduino functions reference:

Reference > Language > Functions > Analog io > Analogwrite