Theory:

     “Wait just a minute... ’58... just let me check in my diary.”
 
     For a minute there was silence. Bepin Babu could feel that his heartbeat had gone up. He was sweating a little.
 
     “Hallo.”
 
     “Yes.”
 
     “I've got it. I’d been out twice.”
 
     “Where?”
 
     “Once in February — nearby — to Krishnanagar to a nephew’s wedding. And then... but you’d know about this one. The trip to Ranchi. You were there too. That’s all. But what’s all this sleuthing about?”
 
     “No. I just wanted to — anyway, thanks.”
 
     Bepin Babu slammed the receiver down and gripped his head with his hands. He felt his head swimming. A chill seemed to spread over his body. There were sandwiches in his tiffin box, but he didn’t eat them. He had lost his appetite.
Explanation:
 
Dinesh was surprised to hear Bepin's question about the 1958 incident. It might be that Dinesh hasn't expected the question from Bepin. And, it took a minute for Dinesh to refer to his diary to get details about the 1958 Ranchi trip. Then, silence fell for a moment. Bepin's heartbeat had increased, and he could feel that. He then started to sweat.
 
At last, after few minutes, Dinesh got back to Bepin's question about his stay at Calcutta during 1958. Dinesh started to answer that he went out of Calcutta twice that year. The first trip was to Krishnanagar, and Dinesh had been there for his nephew's wedding. And, to Bepin's surprise, Dinesh's second trip was to Ranchi in 1958.

Dinesh also mentioned to him that Bepin had accompanied him during the 1958 Ranchi trip. Dinesh's answer almost shook Bepin's confidence in his memory. Bepin felt sad, on having confirmed about the Ranchi trip with Dinesh.
 
Bepin was confident in not having travelled to Ranchi and expected the same reply from Dinesh Mukherji. But, unfortunately, Dinesh had given him an unexpected answer which confirmed Ghose's claim.

Dinesh then enquired about the reason for such a sudden investigation about the 1958 trip. But, Bepin ignored to explain the cause as he did not want to give a chance for Dinesh to tease him. Bepin answered him, saying, "I just wanted to — anyway, thanks." Bepin then kept his receiver quickly with great force.

Shock and disappointment filled Bepin's mind. Dinesh's further confirmation made Bepin to worry more. His head began to whirl. He felt a shiver run through him. He didn't eat the sandwiches that were in his tiffin box. Bepin wasn't hungry anymore, despite the fact that he hadn't eaten for a long time. He was confused.
 
Bepin's sad state.png
Bepin's misery
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Sleuthing An act of investigating an event
2.
GripTo hold something tightly
Reference:
National Council of Educational Research and Training (2008). Honeydew. Bepin Choudhury’s Lapse of Memory- Satyajit Ray (pp. 59-69). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.