Is it normal to get really lightheaded after holding my breath for a few seconds upon standing. hr increased. i experienced it also during chest xray?

Absolutely,thoughnot: healthy in most situations, especially to the point of not protecting yourself, falling, being injured. Just the act of standing drops venous return to your heart, thus heart filling/output & BP. Holding you breath, while doing so, further opposes venous return from both upper & especially lower body; accentuating the basic physics issue of compensating for gravity pull on blood in your veins.