Nausea for no reason??

Hi, I'm a 16 year old female, and I have had problems with nausea for maybe a year or so. I get nauseated on and off, and my mom always blames it on either nervousness, food not settling right with me, it being close to "the time of the month", or stress. When I get nauseated, it always seems to be one of these things, but there are numerous times I get nauseated for what seems to be no apparent reason. For example, I went to Six Flags for a youth group trip, and I was sick all the way there, until I got there, and then I got a little better, but still queasy enough to not want to eat anything. I wasn't nervous about the trip... I was excited, and then I suddenly got nauseated the day we were going. I don't always vomit, unless the nausea is at it's worst. My mom also thinks it's because I have a weak stomach, or that it's a mental thing. I don't know what's causing it, but it doesn't make sense, makes life worse, and all I want is a cure. I have missed out on many things I really wanted to be a part of because of this silly nausea...