Hello,
Have you ever injured this side of your knee, or twisted and felt it sore, or did it just start becoming painful gradually?
There are several things which could cause the symptoms which you are describing and without doing a physical examination it is a bit difficult to suggest what the problem might be and therefore what to do about it!
If you could see a physio or sports therapist they might be able to do an assessment and identify which structures are causing the pain you're experiencing which will help to direct your options for treating it.
An MRI might be useful but it's helpful to know exactly what the problem is, or have a rough idea, because an MRI may not necessarily be required.
I hope this is of some help to you!
Ruth Cheesley BSc(Hons) MSc MCSP
Chartered Physiotherapist