After killing Ikemefuna, Okonkwo is very upset. He doesn't eat for two days and has difficulty sleeping. He was very fond of Ikemefuna and he knew that this boy had been a positive influence on Nwoye and that they too had become great friends. He was also aware that Ogbuefi had told him not to take part in the boy's death, he knew that being there was wrong but his fear of appearing weak drove him to do it.
This sadness upsets him, he wonders when he became "a shivering old woman." He cannot understand the emotions he has because to him almost every emotion resembles weakness and reminds him of his father. He desperately searches for work to do, anything to take his mind off of his sorrows and the troubling weakness he feels. Of course, being Okonkwo, he's already completed all the work that could be done.
Unfortunately Ikemefuna's death is perhaps even harder on Nwoye who takes it very hard and in fact feels that something inside him had broken. This marks the beginning of the end of Okonkwo and Nwoye's relationship as well.