Friends who don't trust you when you tell the truth must have a reason for being skeptical. You say you aren't talking behind anyone's back... and I believe you! But what's your best theory as to why they don't seem to trust you? Was there something you did in the past that they blame you for?
If you don't the answers then I suggest you talk to the one friend in this group with whom you feel the closest. Have a PRIVATE face-to-face conversation and tell her that you're confused about why these girls don't think of you as a good friend. When you talk with the girl, LISTEN more than you TALK. It's important that you really try to hear what she has to say. You deserve the truth and if it turns out that you have been doing something that isn't "good friend" behavior, then see what you can do to change it.
There is a lot of information missing from your question and that makes it hard to determine an opinion of this particular situation. Keep in mind that any answer that you received would only be an opinion, so a little advice might be a better way for you to decide for yourself how to handle this.
To have a friend, you must be a friend. If there is a misunderstanding between you and your friends, then you need to talk to them to resolve it. If they feel that you have "behind their back" said things that hurts them or with harmful intent, it doesn't matter rather you did or not, the hurt feelings are still present. You need to talk to them, see why they feel that way and reassure them of your loyalty as a friend. One is not automatically given trust, one earns it. If you want to be a good friend, earn your friends trust and to begin to move in the right direction, talk to them about this.