Did you look under the skimmer basket BEFORE you back washed? If not, the sand may have already been there.

Just to make sure we are all talking about the same thing (since you are new to pools), here's a Hayward skimmer:


I'm checking, because normally, you wouldn't notice sand in a pool skimmer, upon entering the pool. You might notice sand on the pool floor -- usually under a pool return like this Hayward unit on Amazon:


Sand filters can leak sand after backwashing, if there is a crack in a "lateral", or if something is wrong with the air relief bypass assembly, or if the pump is too big for the filter (quite common, unfortunately!).

If you can take pictures of your pump, filter, & piping, and send them to poolforum@gmail.com, I'll post them, and we'll be able to help you better.