This will completely depend on the type of bond you both share plus individual perspectives. Some bestiees find it comfortable in kissing each other on the lips as a gesture of love while some might just limit themselves to shaking hands or simply exchanging hugs. You will have to find out what is the boundary for physical touch you are comfortable and your friend is. If your friend is an extrovert who doesn't hesitate from indulging in such touches casually with others then it is possible that your comprehension is wrong. Well! If you really think that the touch is awkward and strange and doesn't lie in your normal zone, then you must observe more and try and find out the reason triggering such touch. If she is a lesbian, then she might not come out openly because obviously she will be scared about losing the friendship or simply fearing her acceptance. Do not panic while she touches you. If you really doubt that something is fishy, give her the space and comfort to bring out things to you confidently.