Why does Charlie feel sick inside?

In his progress report dated April 20th, Charlie writes that he “feels sick inside”. He explains that it is the kind of sickness that does not necessarily require a doctor to treat. He says that “his chest feels kind of empty like he has been punched and is suffering a heartburn, all at once”.
Prior to falling sick, Charlie’s workmates had invited him to an after-work party. At the party, he insisted on taking a Coke, instead of alcohol, and danced with one of the girls called Ellen. While dancing, he couldn’t help tripping about, to the amusement of his friends. He then came to the realization that his friends liked to have him around because they made fun of him. He was the laughingstock of the party. One of his friends, Frank, said that “I ain’t laughed so much since we sent him off for the newspaper that night at Muggsy’s and ditched him”, another said, “he’s a scream”. His friends had made similar statements about him, many times before his operation. Then, he had misconstrued these statements to mean that they liked him. On this night, he finally understands what these statements mean, and is totally embarrassed. He also realizes that most of the people he had called “friends” were not really friends.
In his progress report dated April 21st, Charlie writes how he still cannot bring himself to go to work. He asks his landlady to call his place of work to explain that he is unwell and could not attend to his duties. He has not recovered from the shocking discovery he made on the night of April 19th; that “people think it is funny when a dumb person cannot do things the same way they can”.