Stiffness after quitting gym

[akisonlyforu]

gender - Male
age - 21
weight - 75kg.

I joined a gym for the first time in my life around a month back. After the 1st weight lifting day, there was a pain
in back of left shoulder. I took a pain reliever tablet and pain was gone.  I never weight lifted again. After a week , I again developed a pain in left shoulder and hands. I went to a doctor. He said, it was muscle pull. I was prescribed muscle relaxants, pain killers and ointment for a week . I was prescribed 4 weeks rest from gym. Its been now two weeks after I quit the gym , and the pain is almost gone. But there is extreme amount of stiffness , around my neck and in my upper chest. I don't have any other medical problems as of now.

What is the reason of stiffness ? How long it will be there ? How to treat it ?