It very well could be the horses not knowing how to interact with you; you're new to the herd and you need to be placed. It's also perfectly normal. I find my heart racing when one of the horses tries to take a bite of me, or when I've been kicked. It's self-preservation reactions, if you ask me.
I was never around a horse (outside of minor stroking from the other side of the fence) until early last year. I'm now, less than a year later, in the slow beginning processes of becoming a starter trainer (teaching the horse the fundamentals of riding). It's all in your demeanor that matters with horses.
As for me, it has been noticed by many people that I have a calm, yet assertive personality that doesn't propose a threat. Because of this I'm able to safely interact with our worst horses. (Including the monster pony.) I've been bitten (on the arm, the breast, the hand, hip, etc.), I've been kicked (on my lower legs and once on my thigh). Horses have bucked around me and, outside of double-checking to make sure I'm not in the line of fire, I don't give it much thought. Was the bucking horse bumping up out of excitement (
yay food!) or malevolence (meanness)?
By running away from the horse you submitted to her; she's now your boss and will continue to push you around until you gain your ground back. When feeding loose horses in a pasture I feed
all of them at least 10 feet apart.
I suggest your first order of business is this:
Talk to your supervisor. Let him/her know where your comfort zone is, what your capabilities are, and stop letting them put you in such situations. It may seem as a harmless task to
them, but is obviously very dangerous for
you. I think you should "shadow" one of their experienced volunteers for a week or two; stick with them until you get the hang of things.
Just remember this:
Whoever's feet moves first loses. (Equine Trainer Clinton Anderson)
That quote isn't telling you to stand under a horse that rears; by all means keep yourself safe!
My only example is myself with the pony, before she turned sour to every person imaginable. She was notorious for chasing, biting, and kicking people who reacted to her. One grown man she began to chase -- he ran away, so she continued to chase him. Since then she has no respect for him and has continued her abuse. Myself and my boss, on the other hand, would hold our ground as she would pace towards us. Several times she's even stood about 5 feet away and crow hopped (reared up a tiny bit). Not getting a satisfactory reaction from us, she left us alone and wouldn't pick on us.
Respect from the horses doesn't happen overnight. It's something that's constantly changing and needs to be adjusted (the horses
will test each person multiple times; it's our job to justify that we're still in charge). Don't mess with a horse's food, especially if you don't know that horse well, especially rescue horses, because you don't know anything about them. They may be extremely food aggressive and react horribly to your presence. Others could give a crap less.
Again, talk to a superior. Voice your concerns. Don't put yourself in dangerous situations because they're overestimating your abilities.