You say she felt better within half an hour after getting in the ER. So the ECG was taken when she was still experiencing the discomforts.
If, in that situation, the ECG was normal, I doubt that there is something seriously wrong with her heart.
It is good to read that they keep her overnight and do some additional tests to find out what is the exact cause of her chest pain. (although, keeping her history in mind, there is a big chance they wont find anything now either)
I experienced something very similar. For many years, I would experience random but very brutal chest pain at different times, like a great pressure and sometimes a squeezing, along with the left arm, jaw pain. I've had numerous cardiac work ups because the symptoms mimic a heart attack. Thankfully, we know that my heart is 100% healthy. After many years, multiple hospitals, more ER visits than I'd ever imagined, turns out to be unrelated to my heart altogether. After a referral to a rheumatologist where they took a medical history about me and my parents, grandparents, I was diagnosed with Psoriatic Arthritis, mainly Spondylitis (inflammation of the spine), with my sternum, ribs, and thoracic being greatly affected. Long story short, maybe your mother might benefit from a rheumatology referral to see if she may have something similar. If anything it might rule out some similar conditions based on her symptoms. I wish her well and good luck getting answers.