I get that. For me it's caused by postural orthostatic tachycardia syndrome (POTS), which basically means that when I change positions from laying down to sitting or standing, or from sitting to standing I get lightheaded and could faint. On bad days the symptoms of dizziness and dimmed/blurred vision and weakness never really go away unless I get on the floor. Heat and dehydration can make it worse. While I can't say if it's POTS in your case, you should try drinking a lot of water and make sure you're well hydrated. It could be simple dehydration. If that doesn't fix the problem, you should probably go see your doctor.