Occasionally When I Go From Sitting To Standing/walking I Get

Occasionally when I go from sitting to standing/walking I get a pounding pressure in my head and ears and I can hear my heart beating in my head. There isn't really any pain, just tremendous pressure. It feels like my head might explode from the pressure. It doesn't last long, only about a minute or so. Sometimes, but not always, after one of these episodes I will have a headache. This has been happening for several months now and I am getting scared that it may be something serious.