c++ Array MEMMOVE bug

So, what I want to do is placing a 'p' before all vowels, I'm doing that but because of the memmove, the char array is filled with strange symbols.

#include<conio.h>
#include <cstring>

#include <iostream>
using namespace std;

int main()
{
    char s1[256];
    cin.get(s1,255);
    cin.get();
    for(int i=0;i<strlen(s1);i++)
    {
        if(strchr("aeiou",s1[i])){
            memmove(s1+i+1,s1+i,strlen(s1));
            // duplicating the vowel so i can replace it
            s1[i]='p';
            i++;            
        }
    }
    cout<<s1;
    getch();
return 0;
}