Working with a new horse

I just started leasing a horse. When I test rode her and her owner was there she was fine but today when I rode her she was a little nervous. As soon as I got on her she didn't exactly rear up but she tossed her head way back and smacked my cheek with it and her front hooves came about 6 inches off the ground and she started backing up but once she was in the round pen she seemed okay. How can I get her to trust me more? I will be spending time with her for only about two or three times a week. I don't really know If I will be allowed to do ground work with her or not. But I will basicly be taking care of her and riding her for the next month and a half

she needs ground work done with you. ground work doesn't necessarily have to be working on anything other than your connection with her. have her on a lead line and just hang out wth her. give her a slice of apple every now and then. talk to her. the tone of voice more than the words are what calms them. just get her used to working with you and just you. when she is out at pasture try catching her by the halter and standing near her or burshing her slowly and calmly. just do things that are calm and easy. your calm attitiude will rub off on her and she will adore you for it. this isn't a hard problem to fix she just needs to be around you a little more. she'll warm up to you quickly. for now take it a little slower though. she doesn't sound like she is comfortable with you riding her yet without her owner there which means she doesn't feel properly accquainted with you yet. try doing things to explain things to her like let her see the saddle before you put it on. blow lightly on her nose every now and then thats how horses say hello it teaches them your scent. if she blows back in your face gently then you have successfully introduced yourself to her =).