why is heat called the internal energy of the molecules of a body

Dear Student,
We know that internal energy is a function of  temperature .So , at constant temperature the internal energy is zero . 
From the first law of thermodynamics, we know that $$\begin{gathered} ∆U=q-W \\ ∆U=q-P∆V \end{gathered}$$
At constant volume (isochoric process), volume change is zero therefore work done is zero, hence the internal energy becomes equal to  the enthalpy or the heat. At constant pressure (isobaric process) the system undergoes a change in volume either by contraction or expansion and there is some amount of heat absorbed or lost. (internal energy increases or decreases
When an Isothermal process takes place internal energy is zero but work may be done by the system or on the system and heat energy may be absorbed or released.
If a system is at constant temperature then it doesn't mean there will no work done because volume and pressure change.
I hope this explains!
Regards!