I need advice

Hello I'm new to the board and I need advice I'm married and I'm very upset about a situation that has been ongoing since december of last year. My husband is in school and has made a few friends I'm not generally the jelous type but there is this one girl that I just can't deal with, here is the situation since gas is so expensive some of the students car pooled and that was fine with me but when I would called his cell he wouldn't answer and he would say he didn't hear it we we got into a tiff over that (because last semester he called me everyday @ lunch time and when he got out of school and didn't car pool last year)anyway one day after school I was going out of town and wanted to stop by his work to leave something for him well when I pull up there her car is and they were both there alone needless to say I wa Pi**ed off and we got into a argument about it so he says nothing is going on so I blow it off then a month or so ago I was using his phone and I see hottie listed in his phone guess who's # it is? ya hers he says I don't know who put that in there now he has erased her # from his phone but he forgot his phone @ home this morning and she has called him didn't leave a voice mail but her # is on there. I don't want to freak out but what should I do I so badly want to call her and say ok what the he** is going on between the 2 of you?