Weird smell on new jeans

Hi. I'm a newbie here but i thought you guys might be able to help. I bought a new pair black jeans from Sinsay a couple months back. It wrn through 7 seperate washes just by itself so far. I tried soaking it in laundry detergent and fabric softener within the barrel of the laundry machine after it filled up with water and stopped it then, leaving it for an hour, i thought may the smell will go away. Yesterday did this again but the smell is still there. This weird, burned plastic or gum smell. I never even got to wear it because of the awful smell. You guys gavr any advice?