Used Memory rising continuously.

While playing on Minecraft, I noticed I was getting frequent lag spikes, and I have a great computer. After seeing my debug menu, I noticed my Used Memory would rise until it reached max, cause the game to spike, then it would reset to a low number. I tried a search to find my issue.

Video:
http://youtu.be/KrJ3AAcP_f4

Crash Report:
http://pastebin.com/m0d6wpsi