Joint pain when doing weighted squats and lunges

Ok so I can happily do bodyweight squats and lunges all day long with no back and knee pain but as soon as I start to add weight, my knees and back start to hurt and continue to do so for several days afterwards.
Is this to do with form do you think? I always thought I had good form but Im not so sure now..I exercise at home so I can’t even check with anybody..
I really want to start adding weights as I find bodyweight squats and lunges relatively easy now and they don’t challenge me enough, But I don’t want to risk injuring myself and not being able to do anything..
Does anybody have any suggestions as to where Im going wrong or what I can do?

what kind of weight are you using? dumbbells? an olympic bar? Can you have someone tape you and post it so someone here can take a look at it?

Do you have a large mirror in your bathroom? Can you go in there to check your form or bring the mirror out to your workout room? As a trainer, the first thing I would look at is your form. Make sure the knees are behind the toes and the back isn't arching or swaying. Second, I would look at your weight. Where is is located (dumbbells hanging by your side, bar across your back, or bar held in front)? Are you going too heavy to start? Remember that progression is important and you have to progress slowly. Don't increase your intensity more than 10% per week, so if you weigh 100 pounds and suddenly add weight to your squats, you should start with 10 pounds in added weight. Since weights don't often come in 11 pound increments, I'd recommend increasing reps the next week (say from 10 reps to 11 reps), and then go back to the beginning amount of reps and increase weight when you get to the point that your next weight increment you have available is 10% or less (like 12 pounds and back to 10 reps on the second week). Does that make sense? If not, PM me and I'll try to help you a little more.

Yeah, you could do a form check video. My guess is maybe your knees are caving inwards.

Pretty much everyone I try adding weight to, it's like they're learning how to squat all over again, so don't think you're having difficulties that nobody else has had.

Turn your feet out a bit more and see if that helps. If you have no history of joint issues it more than likely is a form issue. If you have someone video you, do it from the front AND side.