I think it's semi appropriate to work yourself into a fit you want. Usually for a more aero or racer type fit. It tends to take strength training, stretching, and tons of miles. It can make enough of a difference that a professional fit done as a beginner isn't going to work a year and 4000 miles later. I'd go the diy route until you're in good cycling shape.

The low back can be a few sources. For me, my low back gets muscle fatigue and pain when I'm weaker, especially up hill. This gets better the fitter I get. Nothing a bike fit will change, just out of shape. I also find less pain/fatigue when I'm doing regular crunches, planks, back extensions, and burpees.

At 23, assuming you don't have a history of scoliosis, I bet it's just muscles telling you to get stronger.

Put in those miles.