My back pain came on suddenly and seems to be more either side of the lower back or left hand side. What could this be please?

Because your pain has come on suddenly it is more likely to be an overuse injury.

Referred pain is that which can indicate problems inside the body but the dual-sided location of your discomfort doesn't match up with any signs that might indicate something more worrisome, so try to keep mobile and apply some Arnica Gel at least morning and night and it should resolve itself soon.

Best wishes

Earle