1999 Chevy Malibu

I have a 99 Malibu and when I bought it I was told that two of the struts had just been replaced. But I had the car looked at by one of my mechanics anyways. And we couldnt seem to figure out why the back passenger one thumped when you went over a bump considering it was brand new. My mechanic didnt have time to take apart to see but the only thing he could think of was that it was a left side strut on the right side but he said that was near impossible.I have the car for eight months now and it has gotten increasingly worse.I know before when a strut was going on my car it sounded completey differnt and was more when I took turns. But this is a loud bang everytime I go over a bump, it almost sounds like its not mounted right but im positive it is. Then another person told me I needed a new strut tower and it was a cheap part and easy to put on. So I called about one and was told that I would have to go to a welder. So im still stuck in a rut and have no idea what is wrong. Could it be my shock and not my stut.I did have someone look at it and they said the shock looked fine and nothing was leaking?