Confusion in understanding temperature change in exothermic reactions The question due to which my doubt arose $\ce{-&gt;}$


  The following system is in equilibrium:
  $\ce{SO2Cl2 + {Heat} -&gt; SO2 +Cl2}$
  
  What will happen to the temperature of the system initially if some $\ce{Cl2}$ is added into it at constant volume ? 


My attempt at the question : Since if we add more $\ce{Cl2}$, concentration of products will increase which will in turn shift the equilibrium in backward direction, which will in turn release more heat. And since the heat is being released to the surrounding, temperature of the system should decrease. 

Though according to my textbook the answer is temperature of the system will increase.

So please do explain where my understanding is wrong.
I think this comes down to what constitutes the system. If we think of the system as the reactants and products in a container, releasing heat will increase the temperature of the system.