It's nothing too unusual about freak bad luck, after all someone has to be the unlucky 1-in-a-million case.

But fortunately PrimeDice provides you with the tools to check if you're just unlucky or were cheated. Give it a read here: https://primedice.com/verify

The simple way to check is before you start gambling save in a notepad the "server seed hash", then set your own custom client seed (and write that down too). Once you're finished playing, hit the reveal server secret button. Copy and paste that server seed into DiceSite's excellent tool:

http://dicesites.com/primedice/verifier

And make sure the results match.  Before you do that for real, claim a few free bits from the PrimeDice faucet and attempt this procedure and make sure you can see how it works.

Honestly, verifying bets is satisfying. You can remove all the doubts from your mind about the site being fair or not.  =)