What could upper left chest pain be?

Last night I was sleeping but I was woken by upper chest pain on my left side. It was an aching pain and sometimes would get intense and was a pretty much constant pain. I had it all throughout the night and it seemed to get worse when I laid on my left side. Sitting up didn’t help either. I took tums and gas x. But this morning it’s still hasn’t gone away completely. I have had this pain at night before but only once. I am a very healthy person (healthy diet and exercise) so I don’t think it could’ve been a heart attack. What could this be? (I am a female)