Poor quality of jeans and no customer service

I recently purchased a pair of black jeans from Pick n Pay clothing, a brand I usually love as the fit and fabric is great. After one wash, this particular pair of jeans pulled loose on the backside, towards the sown-on pocket. It is clear this was a faulty pair, perhaps sewn incorrectly, as all my other jeans from this brand I have had for years with no problems whatsoever. When I took them back to the store the cashier quite rudely told me I could not change the jeans for a new pair as I had washed them. When I tried to explain my problem with quality of the sewing and that this was not usual for this brand she wouldn't hear it. can someone please get back to me on this as I have a pair of R 229 jeans worn once and not able to even fix as the fabric just tears (I have tried to repair). I usually love this particular Pick n Pay jeans but this has put me off buying more as if there is a default, I don't get customer service. (these were bought from Pick n Pay clothing store in Mall of the South). Thank you.