If your back is arching, what you have is too much weight in the middle. If you have some air in your BC (likely) and your legs are light (somewhat likely) then you have your head and feet raised with the weights at your waist pulling down. When you relax, you get an overarched back.
The first thing to do is make sure you have the right weight. A regular buddy of mine changed suits and had to drop 10 pounds of lead. So, do a check. At the end of the dive, empty your cells and get your buddy to pull off weight until you can't sink. Add weight just until you sink with your lungs empty and float with them full. I just adjusted mine yesterday (new suit) and could hover in 5'.
Next you have to distribute your weight. Everyone's different. I put air in my dry boots to hold my trim (I have very heavy legs) and other people throw on ankle weights. Don't add a single pound here -- all you're doing is moving the weight around. The goal is to get more of your weight off your belt. If you don't want to spend $200 on a backplate and then upgrade your BC to a harness / wing, get some 5# pockets and put them on your tank band. Try putting an ankle weight on your tank. I imagine women have to front-load their weights more than men. I don't know. Wait, you're in FL, so that's a lot less weight. I'm going to guess 6#. Is that 3# per side, all ditchable? Man, that's got to rule. I wear 30#, 10# ditchable per side and 10# unditchable. I guess you could put 2# on your tank near the top, each side, then have 2# ditchable at your belt. Is that enough?
You also have to "engage your core" when diving. That means support your gear and yourself by tightening your abdominal muscles. (i.e. suck in your gut!) Even with all that buoyancy, your kit still has mass. Yoga classes help, and besides strengthing your core, poses like downward dog keep your calves flexible and reduce the chances of a cramp.
Stretch before and after the dive. I've never seen anyone else do this. A great lower back stretch is to lie on your back and pull your legs up to your chest. Grab your shins and pull in. Push with your legs, but not hard enough to break your grip. You should feel pulling in your lower back.