Hi Farhana, In this sentence, "or so I thought" suggests that what actually happened was contrary to what they believed was happening. They thought that a number of things were racing through their imagination, but that was, in fact, not the case. It is a natural use of the expression, but I would just add a comma before "or", i.e. "Pretty much just a number of things racing through my overactive imagination, or so I thought." :-)

It sounds quite natural. I would add a comma after imagination. And the phrase needs a verb to be a complete sentence, so I would add "it was" to the beginning. It was pretty much just a number of things racing through my overactive imagination, or so I thought. "Or so I thought" implies that what you were thinking at the time later turned out to be erroneous. In this case, it means that the things you thought were imaginary later turned out to be real.