I agree with Lisa. Adaptation takes a bit of time. Keep riding and you should start feeling better. If not, then look to another cause.

My Luna is the most comfortable bike I have ever ridden. I, too, had a Univega Gran Turismo--great bike! But mine didn't fit me (I haven't had that bike in years and years, but it took me across France when I was 15).

You don't mention your general fitness level. If you have been not exercising lately, of course, your muscles will feel it when you start. If you are exercising differently, you'll feel it too. How's your core strength? Strengthening your core through pilates, yoga, or other back-friendly exercises will help with your cycling.