What Causes Sudden Bouts Of Heaviness And Weakness In Body?

I have sudden bouts of heaviness in the body like i dont have the strength to move on. my whole body feels fluttery and shaky and jittery and my head feels so heavy that it feels like my neck cant hold it up. I feel so week that I feel like i will collaps. and I feel very disorientated. there is what feels like tension type headacre as well. these are sudden event for example icould be walking down the hall way no problems and all and it hits me and I fear I wont make it to the bed or chair to sit dow as I feel I will fall down. which in three cases now I had collapsed as the strength to hold myself up was not there.