母ちゃんも心配でたまったもんじゃない

Question

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.

Answer 1

The expression たまったもんじゃない is similar to たまらない but stronger.

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.