Light headed????

Whenever I’ve been sitting down or laying down for a while and I get up, my vision goes black and I feel like I’m going to fall over. I haven’t ever actually fallen over though. Sometimes, this doesn’t happen. Also, the severity varies; sometimes it only last a second or two but today I had one that lasted around 5-8 seconds, but after it was over I felt like I was dreaming (that probably doesn’t make sense but that’s the only way I can explain it). It only felt like a dream for like 3 seconds before I realized it was real life. I’m not really dehydrated or anything by the way, but I was just wondering what could be causing it, how serious it is, and how I can fix it. Also is it even considered being light headed? Thanks.