Thanks for your query ...
Your words suggests you are having too tight cast leading to compartment syndrome
You should immediately visit your Orthopedic surgeon and get the cast opened and anything that is obstructing the circulation of hand should be removed. The condition requires restoration of blood flow and reduction of compartmental pressure.
Otherwise there are chances of development of Volkmann’s ischemic contracture.
You should use a sling to keep the area still and raise the arm above heart level.
Hope this helps.
Update here about the situation.