Saddle doesn't fit me?

On the weekend I rode using my trainers saddle, which I know is too big for me but usually isn't too much of an issue. Anyway, I was sitting to the trot and felt like I was about to slip out of the saddle, as well as that, it HURT! I can sit to the trot competently so I assume it is the saddle, but if someone more knowledgeable could give me some input that would be great!
Thanks.

It could easily be the saddle.

If it's English and conventionally treed, the tree rails may be just the wrong distance apart for you so the edges bear on the base of your pelvis, particularly if the saddle is old and the seat foam's starting to collapse. It's a pretty common problem for ladies, unfortunately :(