I had a weightlifting session about 4 hours ago, but I'm already feeling achy, is that a bad thing?

Not necessarily. It's very common to feel sore in the first few hours after exercise. Over the next day, your muscles will be breaking down (catabolic state), but that's good -because soon they will start building up again! It's important that you give yourself that rest and recovery time the next day to allow for that regrowth and repair. If you do decide to exercise, make sure it's a different muscle group.