There's no such thing as a "normal" PSA number: many things can influence it, including an enlarged prostate. and as you've already experienced, there can be occasional unexplained variations.
More of concern is any overall trend and rate of change in PSA readings.
I'd be inclined to have another PSA done to see if there is an obvious trend developing.
Request a "free PSA" test as well: this shows the % of PSA from normal benign causes.