母ちゃんも心配でたまったもんじゃない


Another がばいばあちゃん question. One sentence ends in もんじゃない which I understand as,
「〜すべきではない（忠告など）」. But it's strange the verb preceding it should be in past
tense.

Here is the sentence in context.

> 困ったことに、小学校に入る頃になると、俺はチョロチョロと夜中に家を抜け出しては、かあちゃんの店にでかけて行くようになったのだ。
>
> さっきも書いたように周りはスラム街である。
>
> そんなところを、ちっこい俺がチョロチョロやって来るのだから、 **かあちゃんも心配でたまったもんじゃない** 。

I assume the translation would be something along the lines of: "As I came to
such a place and darted around, mama got terribly worried - a thing that was
regrettably no good for her."

It would good if I could understand how past tense verbs can be used with
もんじゃない.

Thanks in advance.

Edit:

I think I have answered my own question and have given an explanation below
with the appropriate translation of the passage in question.




The expression たまったもんじゃない is [similar to たまらない but
_stronger_](http://www.weblio.jp/content/%E3%81%9F%E3%81%BE%E3%81%A3%E3%81%9F%E3%82%82%E3%82%93%E3%81%98%E3%82%83%E3%81%AA%E3%81%84).

So the translation would be something like:

> When I entered primary school, a new trouble developed. I used to sneak out
> of the house at night and scurry through the streets on my way to her shop.
> As I mentioned earlier, the area around my house was a slum. Knowing this
> was no place for a little boy, Mama got worried sick and couldn't take it
> any longer.

