What is wrong with my mineshafter?

I use mineshafter instead of the real minecraft and I seem to be the only one with this problem. My friend also has mineshafter and this doesn't happen to her and I'm so confused. Once I login to my account and start the game in singleplayer, it loads the world and shows the blocks but after 2 seconds, they disappear and all I could see are chests, mobs, water, lava, and people… This also happens in multiplayer when I play on servers but it doesn't do that after 2 seconds. It just randomly does this weird glitch while I'm playing. It's so weird! The last server I played on with my friend, I explained it to another person and that person thought that I was hacking and using some hacking system that allows that player to see mobs and blocks, but in fact I wasn't and I had no idea what that person was talking about! All I really did was directly download it from that site and update my Java, but it won't go away and I can't find anything else similar on the web to my problem! Please, anyone, help?