Knee pain when bending & kneeling?

I have a 4 month old baby & about a month or so ago I started easing back into exercising. I started off slow & now I'm doing about 45 mins. 5-6 days a week. I was in good shape before but did gain 40 lbs. (I have about 10 left now). Anyways, I'm doing 15 mins of running on the treadmill & 10 mins. of incline walking followed by 20 mins. of weights & band exercises about 2-3x a week & the other days I do cardio & strength training videos. I've noticed lately that my knees have been hurting only when I bend or kneel. It almost feels like they're stiff. So I'm not if it's from the running cause my body isn't used to it & maybe the extra weight doesn't help or if it's even from all the kneeling I'm doing changing my son's diapers? I don't feel like I'm overexercising at all or overdoing it. I used to exercise a lot before! Any tips?