So when you pick up on the physical sensation, this doesn't mean it's activated per say, this can just mean you are picking up on the flow of energy.
What I'm saying is that our energy centers are always active to various degrees. There are many in the body, hundreds of minor centers in addition to the major ones.
Through practice, we can sense our energy centers, and this practice can be as simple as focusing on our physical body through taking a time out from being in our head. There are times when this just happens, and we begin to notice either the flow of energy, or the interaction of energy between our energy body and that of other. But this can also include incoming energies, such as eclipses.
If we, or someone else, has the ability to visually pick up on what is taking place, this can help answer the question at the time, as it can be a number of things.
When I am sending healing, I always feel the energy flow through my hand(s). When I am sensing the state of a chakra, or a subtle body, I also pick up on a similar sensation, but it is different. If there is an eclipse, and at other times outside of healing, I will notice the movement of energy.
I felt very connected to her as I hugged her and was teary-eyed. The unexpected thing is that my hands activated—feels like energetic balls in the palms of my hands. I told her this & and we talked about her feelings.
So in your example, based on that interaction with her, you are getting out of your mind, you are focusing on her, and your attention has shifted from the physical interaction (hug) and so on, so you are more open and sensitive to what's going on with your physical body, and any psychic senses.
You could have been picking up on her energy, and that could be a number of aspects, such as a subtle body—the sate of the layer around her. It could be her aura as a whole. It may be you sending healing unconsciously.
I heard it is permissible to direct the reiki energy if I ask their HS if it is ok. If I feel them activate, am I being guided to use them to send healing?
It's always important to ask permission, not to just send someone healing. You can ask higher self, if it's a situation where you can't ask permission and if you feel compelled. I had a client recently here in Christchurch who would experience pain after getting an energy healing, and she has tried different healers. The pain would come the next day in her arms. If someone sent her healing without asking, distantly, it would give her pain. So while we think we are helping someone, we may actually be harming them in that regard.
So I covered your second question above; the sensation can be related to a range of different things. As you develop over time, your intuition will kick in to help you understand what it is more likely about, and like I said, if there was a sensitive there who could see what was going on, they could provide information around what you were doing/picking up on.
Be open to the possibilities of what it may be and then go with your gut. I have this happen to when I am in places, and around people, it's just so common now that I don't think about it much as it's like an every day thing just about, depending on what I'm doing. Like right now, my hands are activated and I'm feeling the energy, simply because of the topic at hand and being aware of them.
Through energy work, which I have mentioned on the forum elsewhere and brought up in a couple of transients' distant group healings, we can help to make our body more sensitive to energy movement through creating various thought based energy forms to interact with our energy centers, and channels. Over time, we just open up more and more, and this has helped me too.
And remember, if you can't feel anything that doesn't mean you are not healing. It's like when a person does a distant healing, and they feel nothing and sense nothing... doesn't matter, it's still working, and the same applies if they are on the receiving end.