Slippage

I just started using my Diva Cup yesterday.  I trimmed the stem (almost completely off) and I have been putting the cup as far up as I seem to be able to.  My problem is that, whenever I walk more than 10 feet, it starts to slip out. I don't feel it when I'm sitting or laying at all --it's just when I move. I tried running and had to stop because it felt like it was falling out.  It feels like how a tampon feels when it's full and starts to slip out.  It's pretty uncomfortable, and I'm trying to figure out if it's normal that it feels like that.  I tried pushing it horizontally back, instead of up, and that seemed to help for a while, but it slowly fell back down.  

I have the smaller size - I am only 20, I've never had kids, and I really don't feel like my vagina is any larger than normal (then again, I guess I wouldn't really know :).
 
So, I guess my question is--has anyone had these problems and, if yes, what did they do to correct them?