Linked list: get node by index has unexpected results - c

Question

This is my code:
#include <stdlib.h>
typedef struct _node {
int data;
struct _node* next;
} llnode;
llnode* llgetindex(llnode** header, int index){
if(*header == NULL) return NULL;
llnode* i = *header;
llnode* j = *header;
if(index < 0){
// negative indexes
while(index < 0){
if(i == NULL) return NULL;
i = i->next;
index++;
}
while(i != NULL){
i = i->next;
j = j->next;
}
return j;
} else {
//positive indexes
while(index > 0){
if(i == NULL) return NULL;
i=i->next;
index--;
}
return i;
}
}
llnode* push(llnode** header, int data){
if(*header == NULL) {
llnode* item = malloc(sizeof(llnode));
item->data = data;
item->next = NULL;
*header = item;
return item;
} else {
llnode* item = llgetindex(header, (-1));
item->next = malloc(sizeof(llnode));
item->next->data = data;
item->next->next = NULL;
return item->next;
}
}
llnode* pop(llnode** header){
if(*header == NULL) return NULL;
llnode* last = NULL;
// if list is one item long
if((*header)->next == NULL){
last = *header;
*header = NULL;
} else {
llnode* blast = llgetindex(header,(-2));
printf("[0]%lli\n",*header);
printf("[1]%lli\n",(*header)->next);
last = blast->next;
blast->next = NULL;
}
return last;
}
I've traced the problem to the function llgetindex; it somehow is overwriting the pointer to the first element of the list and changing it to what llgetindex should return when given a negative index, but i don't get how.
The strangest thing is that calling push works fine, pop doesn't and calling llgetindex directly also destroys the list.
If you can explain why it happens i would be happy.
To reproduce the problem:
create a llnode** pointer and call push function a few times to populate the list, after call either pop or llgetindex using a negative index and check the list, now the first node of the list should now be either the last if you called pop, or the element indexed by the negative index given to llgetindex.
gcc (Debian 10.2.1-6) 10.2.1 20210110