pain or a pain

[Corleone]

Hi,

I was wondering whether someone could explain the difference (if there is one) between "pain" and "a pain". For instance, I have seen the following sentences:

She felt a pain in her shoulder.
She felt pain in her shoulder.

Thanks heaps in advance :)

Corleone

[Alan]

Both possible with no significant difference. However, the countable form (a pain) is more natural in relation to a specific body part.