I had four colon polyps removed, one was 15mm and causing bleeding in stool. I was told I didnt has ibs/chrons/colitis. What caused these polyps then?

Viral field changes. With 4 polyps one needs to know if there is a familial history of polyposis. In addition are the polyps adenomatous or inflammatory. Rare for an adenomatous polyp of 1.5 mm to be symptomatic. If not congenital and appearing adenomatous, the mechanism appears to be polyoma virus transfecting mucosal epithelium and then as various lesions appear if a malignancy arises it suppresses the others.
Genetics. Do you have family history of multiple polys and/or colon cancer. Even if do not, it would be prudent to have genetic testing done to assess your susceptibility to colon disease, especially cancer. It is unusual for an 18 year old to have multiple polyps.
Colon polyps. There are many causes of colon polyps. Colitis like crohns and ulcerative colitis can cause polyps. They could be hereditary (familial polyposis) or could be due to chronic constipation or prolong irritation. Sometimes there is no underlying cause determined. Polyps can cause bleeding. but if polyps are benign without dysplasia or carcinoma, you are fine after removal. Follow doctor's advise.