Back Of The Knee Pain

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Hey all,

I've been getting into fitness in the past few months and have been doing a regular routine of squats and the such with an 8 lb dumbbell. I'm super novice, so starting pretty simple. However on a squat today I went down and something in the back of my knee kind of shifted and now it just feels weird with a slight ache to it. Any idea what I did wrong, and how to fix it? Thanks for reading :)