I've been doing a basic 5x5 type workout since New Years (yes, I actually stuck with my resolution for 5 1/2 months so far!). Each workout begins with either squats or deadlifts, then a mix of other basic moves - bench press, overhead press, pullups, etc.
My strength increased consistently and I put on maybe 5-10 pounds of muscle over 5 months. I was thrilled, but recently my knees began to ache. Sometimes the pain occurs while I am doing the deadlift or squat, but not usually. The pain will typically strike at random later in the day, while I am walking to lunch, or walking home from work. It's not a sharp pain, but deep and sometimes intense - like I have to sit down because I can't walk intense. It soon passes and I hobble on, then I'm walking normally five minutes later. I consult Beginning Strength regularly and try to be rigorous about my form, but I think I must be doing something wrong. What is the most likely cause here? Am I just getting old?
If it helps, I am 33 years old, 5'11" and about 165 lb. I've always been slim and not particularly strong, but I've always worked out enough to stay in moderate shape. This is is the first time I've followed a really structured plan to get stronger, as opposed to just going to the gym to do some lifting and/or cardio, which has never injured me before.