Meditation is different than Reiki. The goal of meditation is to practice quieting the mind. The goal of Reiki is to work with energy.
On the physical level, meditation is done by entering certain brainwave frequencies. Reiki often does so as well.
The person making the video may have been offering a guided meditation that (may have) made use of mudras to affect energy.
Mudras aren't part of traditional Reiki, though symbols are sometimes drawn in the air, so I'm not sure which the video was offering.
Sometimes people use the term Reiki loosely and really are referring to energy work / energy healing.
Reiki is about unintrusion. The energy is meant to flow and go where its needed. It's a conversation between your energy layers and centers and higher self.
So a video forcing energy down a particular path, intended as a generic offering to anyone stopping by, is going to be wrong for some people. Depending how careful the person was about designing the session and keeping this in mind is going to matter.
So firstly, understand this session wasn't necessarily intended for who you are right now in this moment.
The energy system is a dynamic moving system always responding to what's going on internally and externally.
Chakras can be closed down for repairs and forcing them open can be counter to what is needed at that time. Sometimes they are closed down for other reasons and opening them is neutral or beneficial.
Since you were performing energy healing on yourself with little or no training - or being guided by someone who isn't sending energy specifically for you - results are far less likely to be just what you need right now.
Did the video help you ground first? Do you know how? It sounds like some of what was going on had to do with grounding. Pressure in the head is sometimes caused by "feet in the head" - the person is mostly out of their body.
Pressure in the 3rd eye region can be related to energy intensity, energy that isn't quite right for the situation, being too much in the head, and any number of medical conditions and musculoskeletal situations, or something completely personal to you in the mental, emotional, spiritual layers of you.
A Reiki practitioner working on you specifically will start the energy flow and not seek to tell it what to do.
Opening chakras that have been closed many years can cause healing crisises. It needs to be a gentle process, honoring that there was a reason it happened, and that there may be mental emotional spiritual components that needs to be worked through.
Sorry there aren't concretely answers here.
Another thought: Have you checked the comment field? It's possible the video maker discovered trends after the fact and has an explanation specific to what was offered.