Knee weakness after running for some time

Wanted to run this by the board and I'm hoping it's not anything I would need to see a doc about.

So I got tired of being a fatty and I started the "Couch to 10k" app so I could ease in to things and not hurt myself more than the normal soreness that come with it. I'm 7 weeks in and everything has been going great. I can feel my endurance increasing and my recovery time has been decreasing and I'm even down about 10 lbs. I even feel I could run a respectable 5k at this point.

Now I'm experiencing something strange. I starting to have weakness in my left knee. No general pain or discomfort. It just feels like I wouldn't be able to run a shuttle drill without falling over. I can still run my workouts with no change in how it feels. The only time it somewhat "hurts" is getting up from a squat which I just attributed to the weakness and not being able to exert the same strength with my knee.

I'm 37, 230,and I had a meniscus tear in the same knee about 10 years ago that was just cleaned out and not fully repaired. Haven't had any problems due to that and recovered well enough to joint a crossfit program for about 4 years.

Are there any workouts or exercises I can do to strengthen my knee specifically? Or should I just not ignore the signs and just go see an orthopedist?