There are so many different ways of saying something happened so fast that it interrupted what you were doing.

Such as: In an instant I was diverted from catching Sam as he walked out the front door by this blinding flash of light.

or: While reading “________________” it brought to mind something I had wanted to do for a long time so, dropping everything else, I decided without hesitation to do it.

or: It was so unexpected but so very tempting. Giving into that temptation at that very moment seemed to be the right thing to do. So I did and believe me, I surprised myself at how quickly I changed from this quiet, conservative person to this wisp of the will.

I am sure you get the idea. But, as I said, it really depends on how you intend to use it. Give us a little more info and you’d probably get better answers.

Good luck.

If talking though about ‘doing something on the spur of the moment’ you could maybe use ‘impulsively’.

If it was something happening it could be just ‘suddenly’.

Hope this helps a bit.

Without thinking…

…while wrapped up in the moment,…

In no time at all,…

Immediately, while deeply concentrating only on dancing,…

Instantly, by accident,…

With no time to react…

With no regard for the implications, …..

Not realizing what had quickly happened, …

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spur-of-the-mo·ment /ˈspɜrəvðəˈmoʊmənt/ Pronunciation Key – Show Spelled Pronunciation[spur-uhv-thuh-moh-muhnt] Pronunciation Key – Show IPA Pronunciation

–adjective occurring or done without advance preparation or deliberation; extemporaneous; unplanned: a spur-of-the-moment decision.

Synonyms: ad-lib, by ear, extemporaneously, impromptu, improv, improvised, makeshift, off the hip, off the top of one’s head, offhand, spur of the moment, unrehearsed

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so: at the last second, unplanned.

hope that helps!