Had a pain in my left flank for 4 days. When do I call the Doc?

[late]

Hi,
the pain is on the left side, more on the back than the side.But it's off to the side of the back ,uscles that run next ot the spine. It' slightly above the level of the belly button. It's been with me a few days. It bothered almost ot the point of nausea on Saturday. It seems a little better now. I recently started working out in the gym. Could it be related to that? It feels deep. Any ideas? Thanks

[late]

Thanks to the overwhelming repsonse I have received here, I made an appointment to see my Doc manana.