Dr. Visit - When?

Hello! I'm pretty new, and I'm curious about how to tell when I should seek professional help with a sore knee. I've been swimming heavily for a few years now, added in running six months ago, and added biking two months ago. I'm trying to start out easy, but I really enjoy my workouts!

My knee began hurting about a week ago. While I'm still mobile, my knee feels like an overstretched rubberband. I've cancelled all workouts (except one swim - I was going stir crazy!) since then, but it doesn't seem to be improving. Is it time to bite the bullet and see a doctor? At what point should I give up on the restorative benefits of taking a break?

Also, my first 5k is in 16 days. I'd love to be able to race it, but don't want to risk serious injury.

Thank you for any advice!