Random Stabbing Pain Above Left Knee?

Could be anything from some sort of minor nerve pain/ strain to be a life threatening DVT obviously, you need to ask your doctor.
None of us here are qualified to answer and guessing is not appropriate. See a doc.

Welcome to Fluther.

Depending on your age and other medical factors – which you should present in person to a doctor, not to random strangers on the internet – there are any number of potential causes.

It could be anything from blood clots to restless leg syndrome to old athletic injuries which are aggravated by the daily routine of work, and it could even be as simple as improper technique in lifting those weights (which are not extreme, as you describe them), but which could still cause a type of repetitive motion injury.

Blood clots in your leg veins can be a serious medical issue leading to stroke and permanent injury or death. The reason is that since your leg veins are so large, if a clot forms and then releases it can easily pass to the heart and from there to the brain, which causes the stroke.

Since you’re working in a warehouse I’m going to assume that you’re an adult, probably male, and may have an athletic injury in your past. (We don’t need to know details, but your doctor might.) It could be old ligament damage, bone chip/s from an old injury, or who knows what. (Probably not cartilage damage “above the knee” ... but I’m not a doctor, so what do I know?)

DVT = deep vein thrombosis, I assume, @Coloma.

I agree, this is a matter for a qualified medical practitioner and not for random opinions.

@Jeruba Yes. deep vein thrombosis or in lay terms a blood clot.