Monday is an easy place to pin a heavy feeling on. The real cause, though, might lie somewhere else entirely.
A trigger isn't always the cause
A packed schedule can make tension you already carried more noticeable. Maybe the unease started Sunday evening, after a conversation, or with an expectation for the week ahead.
By not settling too quickly on one explanation, you leave room for a more precise picture of what's really weighing on you.
Compare different weeks
Was every Monday heavy, or mainly the ones after a busy weekend? Differences between weeks are often the best clues.