① I'm not a bit tired, so I needn't have a rest.
② Tom's mother couldn't sleep well recently. She was not a little worried about Tom's health.
两个词组在加了not后，意思好像大不相同，请问这是怎么回事?
a bit 和 a little 在意思上非常相似，都表示“一点点，有点儿”之意。它们用作副词，表达“程度”时，修饰动词、形容词、副词、介词短语或比较级，意思相同，可以互换。例如：
He is a bit like his father.
He is a little like his father.
他有点像他的父亲。
The speaker spoke a bit louder in order to make himself heard more clearly.
The speaker spoke a little louder in order to make himself heard more clearly.
讲话人把嗓门提高了一点儿，以便让别人听得更清楚。
但是，在下列情况下，它们是有区别的：

1. 当它们用作定语，表达“数量”时，a bit后要加of，a little 则不用。例如：
She ate a bit of bread for lunch.
She ate a little bread for lunch.
她午饭就吃了一点点面包。

2. 但是，一旦否定，not a bit 和 not a little的意思就大不一样了，甚至相反。
not a bit 意思是“一点也不”，相当于not at all, not in the least之意。例如：
He is not a bit like his father.他一点也不像他父亲。
Are you hungry? --- No, not a bit. 你饿了吗? 不，一点也不饿。
She is not a bit happy.她很不开心。
not a little意思是“非常，很”，相当于 very (much), a lot 之意。例如：
He is not a little like his father.他很像他的父亲。
I am not a little surprised at the news. 我对这个消息深感惊讶。
The boy was not a little worried because he hadn't heard from her parents for a long time. 那个男孩很担心，因为好久没收到父母的来信了。
【注意】有时bit之前，还可以加little，进一步修饰程度之低，如：
I'm a little bit tired. Let's take a short rest. 我稍有点累了。让我们休息一会儿。