Thanks for your question on Health Care Magic.
I can understand your concern.
Since you are having left sided
chest discomfort, better to rule out cardiac cause first.
So get done ecg and 2d echo.
If both are normal then no need to worry for cardiac cause.
Possibility of muscular twitching due to bad posture in sleep is more.
And you are also improved with
muscle relaxant ,this also favours muscular cause.
Another possibility is
calcium deficiency, as you are improved with
magnesium supplement.
So get done serum calcium level to rule out deficiency.
Hope I have solved your query.
I will be happy to help you further.
Wish you good health.
Thanks.