nuances - What is the difference between すみません and すみませんでし た?

What is the difference between すみません and すみませんでした (or すまない and すまなかった)?

If it is what the 「でした」 suggests and すみませんでした is the past form and used to apologize for things done in the past, then what I don't understand is that even mistakes that I did just now is already in the past (I have already done it!). Then すみませんでした should be the correct one to use and すみません should be used only when you are trying to apologize for something that you will do in the future. But that doesn't seem the way it is used. Can anyone please explain the nuance difference?

Edit: Here is the thing that is bothering me:
Anime: キルラキル

(After complementing her clothes 鮮血{せんけつ} about how good it feels, 鮮血 starts to cry)
流子｛りゅうこ｝：「バカ！服が泣くな！ビショビショして気持ち悪いだろうが。せっかくいい着心地{きごこち}だったのに。」
鮮血：「すまなかった」

So he was sorry about an incident (crying) which happened in immediate past (and he was still crying when he said that), but he didn't use すみません. What would be the difference in meaning if he said すみません? Or is it totally inappropriate in this context?

Answer

What bothers you is the ambiguity of ～た form which could indicate both past and perfect. And the worse thing is you have no way to distinguish them in form, in this case.

What you did: すみませんでした

その節はすみませんでした。

What you have done: すみませんでした (~ present perfect) or すみません (~ present)

こんなことになってしまいすみませんでした。
こんなことになってしまいすみません。

Now or future: すみません

すみません。トイレはどこですか？
もし失敗したらすみません。