That is a excellent gain!! Have you been using it as a extender or with the bungee fastened to the leg/around the waist? For how many hours and did you use rest days?
I’ve been using it as an extender for most of the time. I’ve used it with the bungee around the waist, for about a month while I was on vacation. I started using it for about 6 hours daily at first, then moved up to 10 hours daily with breaks in between. The key I think is to use medium tension, that way you have a steady pull and go on using it for much longer, without discomfort. As for rest days, I take the weekend off.