find_word('CamelCaseString', 6) -> 'Case'
find_word('ACamelCaseString', 0) -> 'A'

def find_word(s, index):
    for i in range(index, 0, -1):
        if s[i].isupper():
            left = i
            break
    else:
        left = 0

    for i in range(index, len(s)-1):
        if s[i].islower() and s[i+1].isupper() or s[i:i+2].isupper():
            right = i
            break
    else:
        right = len(s) - 1

    return s[left:right+1]