Use of んじゃない after verbs

Question

EDIT: It seems I have a misunderstanding about んじゃない. I thought it was meant to add "isn't it" to the end of a question. e.g. 田中さんはケーキを食べるんじゃない "田中 eats cake doesn't he?.

According to Tsuyoshi Ito's answer below it seems it can also be used to negate a sentence. I'm not sure what this means. Does it become "田中 doesn't eat cake does he?" or just "田中 doesn't eat cake". If the latter, how does it differ from 田中さんはケーキを食べない.

In summary, how do I know which way to interpret んじゃない?

I've looked at all the related questions on this site but none of them are clearing up my confusion.

ORIGINAL: I'm having trouble making any sense from this sentence:

だってふつうって言ったらふつうなんだから、特別なことしてるんじゃないもん、ねぇ。

After all, when you say you're ordinary, you're ordinary, so it makes you special doesn't it?

I don't really know how to interpret じゃないもん. Is that making my translation go wrong or is this sentence really as silly as my translation?

Answer 1

The difference between the two translations you're thinking about here is the question mark.

んじゃない is just a composite of ん/の　which is used for explanatory purposes and じゃない。(んだ/のだ when its not negated)

With no question mark or tone it only negates the sentence and signifies that the speaker is trying to explain themselves.

When you add the question mark as in the grammar you refer to, 　んじゃない？　it gains the confirmation seeking meaning "isn't?" it etc.

Answer 2

～じゃないもん is a colloquial expression for ～ではないもの. The もん/もの here is used to express insistence.

Your translation is incorrect at least because the negation in じゃない/ではない is missing. I do not know about the rest because of the lack of context. I am particularly unsure about the correctness of any of the occurrences of “you” in your translation.