A common problem with new divers. We're all conditioned to spend pretty much all of our waking time in the "vertical" position ... and when we begin learning horizontal positioning in the water it tends to feel like we're standing on our head. Some people have more of a problem with it than others, but I see it commonly. It is, by the way, one of the reasons why so many instructors train new divers on their knees (more comfortable orientation for the new diver), and why you see so many new divers maintaining at least some degree of verticality when they're swimming. Unfortunately, the side effect of that is a much higher level of effort than you need to put into diving and therefore higher air consumption (due to the extra exertion).
All joking around aside, these prior posts have the right idea ... you need to "reprogram" your brain to be used to functioning in the horizontal position. Some dry-land exercises can help with that.
As for the "leg heavy" sensation, adding weight won't solve that problem ... moving some of your weights higher on your body probably will.
... Bob (Grateful Diver)
NWGrateful nailed it, as usual, before I even read the post. Hi, Bob! A couple additional tips: If diving single tank, mount it higher so the valve hits you in the back of the head (really). Diving dry, get a little air into your feet to help them get up higher. Do what you must to get the balance point at your hips, so that you can hover horizontally, head down, or feet down, without making any effort other than to tuck slightly.
theskull