Sorry to hear that you're not happy with your results. I suggest you wait a couple of months to see if your face doesn't return to normal, then talk to your injector about your concerns. I don't recommending using a facial exerciser, since that can actually contribute to the wrinkles Botox helps to smooth away.
Very difficult to answer your question without pictures. In general Botox injections in the masseter muscles will cause a decrease in the volume of the muscles but it is usually a temporary effect. As you are using the muscles for mastication it will bulk up again. 5 months post botox injection is not long, so it may take few more months for the volume to get restored.
Thanks for your question and I am sorry that you are having the concerns that you are having. Injections into the masseter are very technique dependent and it seems that your injector was skilled in reducing the masseter for you in the past. Now that you are having a concern, it would be really important to review the before and after pictures, as what you are describing is not something that we hear about or see too often from Botox injections in this area. I am wondering if you had some volume loss in the first place which is now something that is becoming more pronounced over time and that the jowl is there as the masseter is made smaller. Please consult with a board-certified dermatologist or plastic surgeon to determine what is going on and what the best treatment options are out there for you now, including the use of RF energy based devices if this is deemed appropriate.