I jumped really high on a trampoline and as soon as my left knee hit the trampoline it started crackling and i am now limping with a bit of swelling.

Many possibilities: There are many possibilities of knee pain, from strains/sprains of soft tissue structures to fractures of the bone. It is difficult to assess your situation with the information given, but the best advice would be to follow up with your doctor for a complete assessment. They may need to do x-rays or an MRI to better assess you. Take great care!

Ice, elevation,rest: It may be a sprain. If not better see your doctor, .