I think we have sprung a spring

I noticed right away one side of the bed was thicker than the other but assumed it would all just level out with a bit of time...4 months later it has not and there is a bulge coming out on the side of the mattress. I am told this is because we did not purchase the box spring but chose to use one we already had.... seems odd to me as only 1 side has done this and both sides sit on the box spring. Sealy just going on a good reputation from the past and I believe their quality of product has gone way down hill. Not at all satisfied with their response however store we bought from has offered to sell us a whole new set and will offer a credit towards previous purchase..