is użyłem a perfective form? as I wrote 'I was not using anything' presumably I would need an imperfective form in this sentence structure.
Yes it is. Well, I believe imperfective is most often used in negations, but it's not a set rule. And sometimes it's hard to see and explain the difference.
I'd say "Nie użyłem niczego" (in this particular situation we're talking about) and "Nie używałem niczego" (during that period of time we're talking about)... at least I think so. Really hard to grasp a difference with this particular example.