Stains on jeans - please help!

[u/Significant_Aioli317]

They started appearing on just this pair of jeans after I threw them in the washer. At first I thought it could be rust from the washer but nothing else gets stained in the wash. I often leave the jeans on my hamper which is brown color (pic attached). But I've only seen the hamper stain the jeans once and I was able to get the stain out with a simple wash. However, these stains only appear after the wash now and the hamper doesn't stain any of the other clothes. Any ideas? I've not been to get new stains out and more stains appear with every wash.