What are these stains and how do I get rid of them?

Does anyone know what these stains are from and how I can get rid of them? I tried laundering after applying Oxi Clean and Spray-n-Wash. Neither worked.

I thought it might be a bleach stain, but I'm not sure if it makes sense for bleach to create a mustard-colored stain on off-white pants.

I wore these pants just once and am pretty bummed about the stains. Any recommendations are appreciated!
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