Working out again?

Meg

So I haven't worked out much since before my first son was born. It's been about 3 years. I went to the gym on Monday and did some bodyweight squats. My form was good (I used to be a personal trainer) but my knees have been hurting ever since...do you think it's just because I'm out of shape or because I'm heavier? Anyone else working out yet? How's it going?