Thank you for your question. Although without being able to examine you I can not offer you the specific advice on diagnosis and treatment that you need, but I would try to provide you some relevant information about your health concern.
Mainly, your symptoms indicate two possibilities here. First, the possibility is of heart related as you have swelling in legs, rapid heart rate and low BP. Secondly, as you have back and leg pain that may originate due to slipped disc or disc prolapse conditions and symptoms may originate due to nerve root pressure, pressure on the spinal cord and pressure on the tail bone. Common symptoms could be paralysis of muscles, pain radiating to the arms or legs, leg pain and numbness, tingling, or weakness. .I would recommend you to see neurologist and cardiologist who can evaluate the details of her case and could better determine the insight of your situation. Hope this helps. Take Care & Regards.
You are not imagining a drop in BP, which causes the dizziness. You need to wear a halter monitor for your heart to see what is happeining with your heart. Don't know why your legs are swelling, but you need to see a cardiologist to evaluate all these symptoms. Drink lots of fluids. You can get medicine to up your BP, but the heart might be the cause of your BP drop.