Then why not?

This may seem like a silly question, but I am creating a new language for a novel I am writing and would like some advice if it is possible..

In this new language there are different words for the word 'not' depending on which tense the sentence is in..

So in a sentence:

We had conversations before so I reckoned why not?

(Excuse the lack of commas and such, if there should be any - still learning)

The first part of the sentence is in the past tense.

In my new language the word not in all the tenses are:

Can (present)

Mhen (past)

Ran (future)

So the question I'm finally getting to:

Is the 'why not?' Considered to be past tense also since the rest of the sentense is?

I realise this isn't your usual question, but it would be very helpful if one of you good people can advise me on the matter.

Thanks In advance