Ever since i went back to the gym my urine had been dark brown. what does this mean?

Urine can be dark for many reasons. The most common reason is dehydration. With going back to the gym you definitely need to drink more water. But other causes such as jaundice or infections or food that causes urine to change color is also possible. If persistent after continuing to drink increased water, please see your doctor.

If dark brown all the time could be blood, Depending on what you call dark brown, If after exercising then likely dehydration and urine is concentrated