Hello,
What I feel is that during any cardiac issue the left hand will not become stiff but it will be painful and difficult to lift up as it will feel weaker. Since the cardiac profile is assessed which turned to be normal then it's not related to cardiac.
Next, the tightness you mentioned, how is it? like you feel pain,
tingling,
numbness kind of symptoms? of its just tightness of muscle?
If you get these symptoms you need to figure out what is the nature of the tightness and what is the nature of the pain?
if you describe the pain and tightness nature than we can come to a proper understanding.
Also, for time being you can use hot water fermentation over the neck region and do some simple exercise for the neck and upper limb. But please make sure you figure out the exact nature of the pain if at all you get in future.
Hope I have answered your query. Let me know if I can assist you further.
Regards,
Jay Indravadan Patel, Physical Therapist or Physiotherapist