Firstly it is unlikely to be over use. It sounds like there is Denis in the knee, either meniscus or even a chip of bone. This would limit the extension and cause the sudden giving way feeling. This can be caused be Truma (direct bang) or a twisting of knee. It is also possible there is ligamentous damage. Have it checked again by a sport physio or sport chiropractor. Preffably see if they do MRI referral.
Saying that it might just be soft tissue, although sounds unlikely. To rule this out try ice and heat twice daily for another week and buy some tube grip for knee at boots. If your still getting locking and giving way after this time- you need a second opinion.
Good luck