Why did our prophet used the word “maii” ( which is a word related to gut) instead of using lev( heart) ?

Shalom, Thank you for your follow up question. None of the classical commentators on this verse comment on the use of the word maii. This is because it is a very common word used in the Tanach, that generally does not need explaining (just as if the word “lev” had been used it would not have needed explaining). However, the commentators in Jerimiahu chapter 4 comment that the word “maii” is used when referring to children, as children grow in the womb – one's innards. Others explain that one's heart is aroused through ones desire, that one has power over to change, whereas one's maii (gut reaction) is caused by fear or other natural emotion that one has no power over. I hope this is of some help.