3months & 1week pregnant with twins. Everytime I stand up I get light headed & dizzy I have eaten food n I just lay down. What could it be that making me dizzy?

You. You may be having orthostatic hypotension, which means your blood pressure falls when you stand up. Twins increase the demands on the cardiovascular system. Some women may have heart failure during pregnancy the risk goes up with twins. Many women experience fainting or syncope with pregnancy as their cardiovascular system adapts to the changes. An examination is in order. Has your doctor evaluated this complaint?
Dizzy. Are you drinking enough water? Young be dehydrated have you gained much weight so far? I do not think that the bulk from the pregnancy is big enough to be pushing on the lower veins, reducing the preload to the heart. At this point I woulda me sure o was well hydrated and if that does not help check with your ob/gyn when it is convenient good luck and feel better.