It could certainly be that the sensation is pleasurable on her teeth.... However, many cats enjoy plastic for other reasons as well. Plastic bags are often coated in substances that are actually attractive to cats. Boredom could also potentially be a contributor....or even just the enjoyment of the crinkly sound bags make. Plastic unfortunately can be a potentially harmful toy for a cat. There is always the possibility that they may swallow a piece or pieces and it can become an obstruction in the gi tract. If your kitty does it for the simple fact that they are just drawn to the material, then the best solution or advice I could offer would be to pick up any bags immediately and either get rid of them or keep them out of their reach. If it is a matter of boredom, there are many different resources available online to get ideas on enriching the environment. There are a wide variety of different toys/puzzles/activity centers that may distract and break your kitty from this habit. Scratch posts, cat trees, and perches are excellent. If it's for the pleasure of the feeling on their teeth - gentle teething toys may be something to consider. Most major pet stores will have a small section of toys dedicated to chewers. The crinkly tubes that fold up/expand might be fancied by your kitty. They make a wonderful crinkly sound. Anxiety is yet one another possibility for chewing on plastic, it's similar to us chewing our nails. The cure for that generally consists of trying to determine what could be causing the stress and either removing it or helping your kitty cope. Pheromone diffusers or sprays are a wonderful aid in many cases. Feliway is a top choice. Try to avoid pheromone collars though - most are not the break away kind and can be choking hazards. There are a couple of medical problems that can and have also been correlated with plastic chewing. Dental related issues can be a factor and last but not least there are some diet deficiencies that could potentially cause it as well. A call or visit to your vet especially if it is a newly developing or evolving habit isn't a bad idea. At the very least they could check the mouth and make sure the gums and teeth are healthy and there are no symptoms of diet related problems. Best of luck to you and your kitty! Remember to pick those bags up and hide any thin plastic. Eating the wrong piece or too much could be a very serious situation that can potentially turn into your kitty needing an emergency surgery. I know it's a pain to be so careful but doing the opposite is just not worth the risk. Good luck to you two!
Well, be careful if you buy treats/supplements. I would steer clear of anything that your kitty has to consume. Honestly the only thing we ever really recommend aside from medication (which only used if absolutely necessary) is feliway for anxiety. It is available in spray, wipes, and plug in diffusers. It is kind of pricey, but we've seen great success with it in certain cases. Supplements or other things can potentially cause secondary problems. :/
Yes, All my cats were seresto collars and have been vaccinated within the last 6 months.
Therefore, I have become diligent about removing any or all plastic bags and put them away in a closet or cabinet out of their reach. I agree that they are coated with a substance that is attractive to cats.
Snowhite525
gave you some excellent advice. 
