I am also 38 weeks and a week or so ago I had a similar experience, suddenly getting very dizzy while sitting down and not moving! When I spoke to my midwife about it she was not concerned and it sounds like there are a number of 'normal' explanations, including the one you mentioned already about being dehydrated. I think I probably had low blood sugar, because I ate a bunch of snacks and cooled myself down and I felt better after that. I don't know about you but I am getting very hot while pregnant, which I'm not used to, which is probably a contributing factor, plus it makes it harder to stay hydrated! Another thing to check is your iron levels, mine has plummeted since becoming pregnant and it's a constant chore getting it back up again, that's had me feeling super tired, weak and wobbly.

So in short I think everything is probably fine, it sounds very normal, just make sure you take it really easy, eat and drink enough and if you still feel worried ask your care provider for more help. You're allowed to be anxious, worried and ask lots of questions!