Does costochondritis generally get worse over time?

Should get better. Costochronditis usually gets better with time, especially if the cause (such as coughing) gets better.
It is unpredictable. The problem is not serious just painful. It is rather unpredictable but usually improves eventially. Treatment with medications like Ibuprofen or heat over the area may cause improvement. Sometimes we inject the area with medicine to relieve the pain. Of course, exercise may temporarily make the pain worse and rest may help it.