ok, so now rei's nearly 6 months. all of her adult teeth have come in nicely. however, one of her front bottom tooth didn't fall out. It's all grayish/yellowish and there's no adult tooth instead. all her adult teeth are so white and big, but this one in particular is worse everyday, tiny and almost rotten, like it wants to fall out but it's not even loose. my big question is where is her adult front bottom tooth? could it be that she was born without it? or maybe won't come out until the pup tooth falls out? should i pull it out? please help!