Has anybody else had any problems washing cotton curtains?

Any tricks or tips or what avoid as to how to wash cotton curtains or drapes?

I bought brand new Ikea white Merete 100% cotton curtains and washed them before I hemmed them to prevent shrinkage in the future.

I rented out my place a couple of years ago, and the tenant sneaked a cat in even though that was forbidden in the lease. There were all these cat hairs on the bottom of the curtains so I had to wash them. But, all the curtains shrank so much I can't re-use them anymore and Ikea doesn't sell those same curtains in that size anymore to replace them.

To avoid that scenario again, I thought it would be better if I washed these brand new curtains beforehand so they would shrink now vs shrinking later after I had hemmed the curtains But, washing these brand new curtains was a huge mistake.

When the sun shines through the curtains, you see all these splotchy yellow marks. I have no idea what caused this, if the metal in the grommet rings somehow caused all this or if it was something else?