I A Female, Aged 54, Non Smoker And Have Had

I a female, aged 54, non smoker and have had four distinct episodes of sharp pain on the right side of my chest - roughly under the collar bone at a height of mid sternum in the last 3 weeks. The pain ( a squeezing sensation) is sudden and increases in a matter of minutes to then disappear as suddenly as it appeared. It normally happens when I'm at rest and does not increase with movement, coughing or breathing.
I've had an xray and nothing showed up.
Thank you.