I had an experience last night on my bus...this man got on my bus went and sat down...then a short time later came to ask me a question..he then stood behind me a bit....and as he stood there it felt as if he were inhaling my energy....I could literally feel my energy leave me and transfer to him....it didn't hurt...but I was worried what that meant and would do to me since I was driving...so I tried to stop it....I pulled my energy back...felt it slow down...felt really odd....I never tried doing that before...he got off my bus shortly after....but it kinda freaked me out...lol....and I don't get freaked out that easily...lol....I think your worrying a bit too much...I think energy transferance is quite natural....I've felt it flow both ways....I had another one where a young woman was standing next to me on my bus while I drove....and I felt her GIVE me some energy....I wasn't drawing it in I either...it just happened...
if you do not want to leech other people's energy, you have a good chance of not leeching! Intentions are strong forces.
I do like to make double sure though, so I simply pray for protections - as perfect as possible! Protections for myself, against negativity in the outside, within or coming through me. As well as protections for the outside against me!
If you have not already - you can also visualize, imagine, pray, or ask for connections to healthy, divine Earth energies (usually through the heels), and to be connected to other divine, healthy energy sources (through your chakras etc.); as much as necessary, possible, available and sensible, and as frequent as necessary, possible, available and sensible! If you do so, you are connected to energy sources far greater and purer than any energy you could (accidental) leech from a person.
It will then be much more likely that people will leech from you (but you can replace that energy relatively easily with your connections (as described above)).
Love and Light!
Being aware of it is half the battle. I would never intentionally do this, though I did question it several months after joining this site if I was or not. I am still learning each day, but sometimes people are sending you this too. So the thought of blocking yourself is not really the best idea if you want to allow other to send you the "good stuff". I try to change the negative to positive and send it back out. There are many different opinions, but this one works best for me.Intention is powerful.
Your surroundings are important for understanding what is happening around you too. I had to strip it all away and start over essentially. I had to rebuild with the new information I obtained through my awakening.. and this is not always easy to do.. but worth it. It all comes back to what you intend to dowith it.
Love & Light
I don't think this exact thing has happened to me, but I have a couple of thoughts.
1.) It is possibly a coincidence, especially if it only happens rarely, but as you seem to be seeing a regular pattern, this may not be the case.
2.) I like to spend time in nature not just because it is relaxing and I am not around so many people, but, if I sort of concentrate, I can pull energy from the ground (and I supposed the plants around me) and feel energized again. I don't do this often, because I do not if there are any consequences to the earth (flora and fauna) when I do this, but sometimes, I can't really do anything else to keep going and I don't think I am hurting anything. Anyways, this feels like a sort of natural thing for me to do. Maybe you have something similar with other people's energy; you subconsciously reach for it when you are tired (emotionally or physically). Maybe you could try to go out in nature away from people and try to grasp energy from there at least until you learn what it feels like. If you learn what it feels like to pull energy from something and then stop pulling the energy from something, perhaps you can be more aware of how you can control this particular empathic ability. If this is what you are doing and why you are doing it, you will probably feel really tired for a while after you "cut yourself off". As Goodenergyhealing mentioned, I'd recommend trying to find other non-leaching ways of getting energy so that you can still function in everyday life.
I don't know how it is for other people, but I stand outside in nature and get myself in a sort of meditative state. I stop focusing on inside myself and shift my focus deep in the earth. Then I pull that energy up through myself up through the feat and out through the head and hands. I usually feel a lot more energized after doing this.