You need to clear my_byte[] before each card reading otherwise you will get the wrong data as bits from the previous reads are still in there. since you only shift 5 bits, there's still 3 bits left over from the last read.

Also, the line:

printf ("%c", my_byte[j]+48); //i add 48 to the char so that it would be a readable character

why do you not put something like:

printf("%c",my_byte[j] + '0');

or more simply

putchar(my_byte[j] + '0');