Oh, no! So frustrating.
I think it is probably a combo of fabrication and poor manufacturing (or the choice to manufacture this style in that fabric in the first place.)
The fast fashion fix is great for help ing us identify new shapes and colours we'd like to try at a higher price point, though. No denying it.
I had a similar experience yesterday with a new sweater from Winners. Also very inexpensive and I had no expectations really, but I didn't think it would stretch and bag out from a day of lounging on the couch!!