B.L.
B.,
I am a local Massage Therapist and see this often in runners. Sounds like you are experiancing strain to the ligaments and tendons from training for your 5k. Often paying close attention to your stride and the position of you foot as you come down can change how you knee is impacted. Make sure you are stretching both Before and After your run and take time time to walk with good posture as a cool down.
Using ice follow by heat (10 minute each) a few times after the activity may aslo bring down the discomfort. Of course getting massage will help you too.
Email me if you want (____@____.com), I may be able to help you out to keep things lose so you can achieve your 5k goal without worry.