Glitch? Prank?

I was playing minecraft quite a while ago, when i first got it in fact, i was playing on a private server my friend had made, he left to go eat and i continued working on our mansion, everything seemed fine until i heard an explosion from the northern wall. When i had gone to check the northern wall more explosions had erupted around the mansion, the explosions were continuous however the mansion had made a stand and when i check all was well on the outside, when i went to check the inside i had tried to open the door but a chat message had stated that it was being blocked so i tore through the wood to find the inside had been demolished completely. this had angered me because it had caused me to think it was a prank from the same friend who went to eat. i had started flinging cuss words at him in the chat until this chat message had popped up: "Stop, we all wear the mask." this had freaked me out considering that i had been alone, the message was spammed and when it had stopped more chat messages started sprouting up such as: "Alone, aren't you alone, aren't you alone," and: "The only one, you aren't the only one." I had been informed of planet minecraft, but never bothered to ask until today, the chat messages sprout up at random everyday. So i come here to ask, is anyone else having this happen? If so PLEASE help, the messages are becoming continuous and it is lagging my computer.