Nearly passed out after getting out of bed, why?

Ok so I just moved in with my grandparents, and at 3 am my grandmother wakes me up to tell me my grandfather fell out of bed and can't get up, so I quickly got up, got dressed and went to help. I lifted him up into his bed, which involved a lot more strain than I thought it would, and after I was done I nearly hit the deck myself. I was feeling very dizzy and lightheaded, and my blood pressure definitely went down a couple notches.

After having a few sips of water and lying down on the ground with my legs up for a couple of minutes I felt fine, but there was a bit of a scare for a few seconds.

Anyways, I usually have near perfect blood pressure, I've never passed out, and as far as I know every single organ/system works just fine so I find this extremely odd. Although I have not been very active in the past couple of months, and particularly yesterday I spent almost all day in bed, so my guess is that this happened because of a lack of exercise and sudden unexpected strain just after getting up, is that probably all that happened, or is there another probable cause? Because odds are I will have to get up at 3am to help my grandfather again in the future and I don't want what happened to me to happen again.