No it’s not necessarily because of diabetes. It’s called Orthostatic Hypotension and it just means you have a temporary or chronic condition that relaxes your blood vessels too much so that more blood is in the lower half of your body than the upper half when sitting. When you stand up, there’s a drop in blood pressure that drains a little blood from your brain and causes you to get lightheaded or in more severe cases, faint. It’s a similar condition to what pilots get when they’re pulling high g turns that pools blood away from their head. An easy solution is to keep your blood pressure up by squeezing your bottom leg and butt muscles as you get up to keep blood in your head.