Hacked?/Trolled?

A month ago, in April, I reported about not being able to place command blocks. Now I have a different problem.

I logged back into my server and as I was building the main entrance to the server, I found repeating command blocks (the purple command block in 1.9) scattered everywhere. They had no text in them, which was good for me.

Then there was this awesome house not built by me (because my builds suck). It looked like it was pasted right into the world. It looked like that because:

1. It was in a lake
2. dirt was blocking the main entrance
3. water source blocks around it do not flow into the empty corners outside

Because of all these things, I think either my account was hacked or someone joined the server without notice, OPed himself, then trolled me.

Or this is just a bug. Or an insect (sorry for the bad pun).

1. I did try restarting my server

2. I can't directly see OPed people, but I'm the only one I used the /op command on. This means no one could have gotten a command block unless they hacked into my account and OPed themselves.

So maybe someone hacked my account to troll me.