Just had the rear springs replaced by a mechanic.
Not sure if it has to do with the work done, but now when the car is full, 2 adults/2 kids, the car makes loud thumps in the rear right side over even the littelest bumps.
When it's just me driving and no one else in the car, it doesn't thump.
I'm thinking it could also be the rear right ball joint/control arm...?
It's a 2014 Mazda2 with 60k.
Any ideas?

 

Have you rear suspension checked by a professional shop.
Sorry but the suggestions would be the complete list of all the rear suspension parts to check.

 

Welcome to the forum

 

My question is why the springs would need to be replaced unless they were both damaged? I would also hope that the mechanic and shop that installed them looked at the rest of your suspension? On a far reach if the spring were incorrectly indexed I suppose they "may" have some adverse effect but you would have more likely seen it in the rear not sitting perpendicular to a straight line of site comparing?
Bottom line is you need to get the rear suspension inspected.