If they hurt the next day, and not while you're running, it makes me think it's due to inflammation. The inflammation could be due to doing too much-too soon, running on too-hard surfaces. What didn't used to be 'too much' for you may be 'too much' at the start of the season (and a year older ;-)) .
Try wearing compression leggings or capris (I use CW-X) while you run to help support the knee aread and/or icing your knees after running (to keep the inflammation down). You can buy specific ice packs meant for the knee (they are donut shaped, with a hole for the patella, and have velcro on to keep them in place).
The best running surfaces are tracks (like at a local HS), but they can be boring. Asphalt is cushier than sidewalk cement, but be careful: I developed IT band problems from doing speed walking on the sides of the road, not thinking about the grade that gives that part of the road a downward slant.
You also might try an anti-inflammatory pre-or post-run smoothie (I've been using these lately, and they seem to help): frozen pineapple (has anti-inflammatory properties), ginger (ditto: I use whole organic ginger root, with peel on, just washing well) with whatever liquid and sweetener you prefer (I use either Silk coconut milk and water or coconut water, depending on whether I'm using more as a 'meal' or as a lighter snack).
