Oh dear. Take some painkillers and rest. You could try ice, but given your previous issues, may be wise to avoid (at least if not properly wrapped in a towel
)
The MRI will be to diagnose what is/was wrong - regardless of the new injury - this is a good thing
No one here can really answer what is wrong with your knee, only a doctor can do so, from the examination and imaging
We can just offer support once you know. I am guessing you are in your teens so this is 10x the case - doctor + parents
Good luck
