Try to just vac the sand to waste, it could be that it's the same sand getting recirculated back into the pool. if your filter had a little water in the bottom over the winter, it could have crushed a small amount of sand that now passes through each time.
Or, you could have a cracked lateral
or you could have a bad fit where your valve meets the standpipe - check for play there, you can add a couple of wraps of electrical tape to help seal it better.