it may be my imagination, but ...; I may be imagining things, but ...
Used when you suspect something might be just your imagination, often followed by a tentative observation.
It may have been my imagination, but his voice sounded like it was trembling.
思いなしか、今日はいつもより道が混んでいる気がする。
I may be imagining things, but the roads seem more crowded than usual today.