“Blood rushes to my neck/face/head when bending over, get lightheaded when standing back up?”

Over the last few weeks, every time I bend over such as to tie a shoe or pick something up, blood rushes to my head, making my face, eyes, and neck strain from the pressure, and when I stand up, I get lightheaded for a few seconds. I also feel like my throat is constricted. Even when not bending over, I feel like my face is somewhat bloated--my cheeks are puffy, my eyelids are droopy, and I have bags under my eyes. I also tend to have an urge to cough when I stand up, but it's a dry cough. Sometimes just coughing makes me a little lightheaded. My blood pressure is normal. My doctor is stumped. He ordered an EKG and a chest x-ray, did a blood test for thyroid function, an orthostatic test, and all is normal. He has no idea what can be causing this. I think it feels like blood flows freely into my head, but is constricted coming back out like something is constricting in my neck, but he dismisses this for lack of any explanation for it. Any ideas?