Hi tabithakristen - Your description of the problem is one of the more accurate ones I've heard, and sounds very much like you suffer from orthostatic hypotension - basically a failure of the baroreceptors in your body (which are designed to refexively increase blood pressure and heart rate to compensate for the effects of gravity when one stands up) to do their job. This can range from the occasional and mildly annoying to a real nuisance and, as you've noted, a potential secondary danger from falling.
The short-term trick is to avoid actually passing out. The solution, usually, is to get up slowly, stop, and wait for your body to adjust to the new position. This can be tricky, however, because our natural tendency is to get up and do something. Also, there can be a lag before the pull of gravity causes the blood to drop from your head down into your abdomen and legs, so you sometimes may get up feeling fine, take a few steps (or more than a few) and then get the drop.
While the condition commonly occurs during pregnancy, those who have it all the time may actually, again as you have observed, experience it less when pregnant than otherwise.
If the problem is becoming more severe with time there are any number of possible explanations, most of them in themselves fairly benign, but still well worth looking into. If the condition is the result of a provable failure of the baroreceptor reflex, there are a couple of different medications that can be tried which might relieve the symptoms. First, though, usually, a tilt-table test is performed, which is probably something you wouldn't want to have done while pregnant.
If you can work out some way to have an exam specifically for this problem when not pregnant, I would advise it. Failing at that I can only suggest - very strongly - that you pause after standing and start moving cautiously at first, as big a nuisance as that may be, in order to avoid a possible fall and the risk of whacking your head or otherwise injuring yourself. You may never actually "fall out", but the potential is there.
It seems unlikely that the perinatal murmur would be related, but again, it's hard to tell for sure without a neurocardiological workup while not pregnant. There's probably something you could be taking that would help reduce or eliminate the problem.
Wish I could give you something more, but lacking health insurance it's going to be tricky. You might comfort yourself with the knowlege that while it's a very unpleasant sensation it is almost always benign - so long as you take your time in getting up and moving.
Good luck to you.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist