Did you use a free VPN when playing on FJ? If you use a free VPN then the IP might be used by other person and you are suspected to have multi accounts. Indeed FJ should give you more proofs about which accounts is connected with yours, but to be honest I dont think that FJ will scam their players just for that small amount of coins.
This is what im supposed to say that FJ wont ruin its reputation just for a small amount of money and regarding on the op situation he might used another ip when logging in his account and FJ found out that activity and since they do have strict implementing their rules this situation does really happen.