Vyvanse + Zoloft = weight gain......add more meds? or reduce meds? or what?.....

I have been on Vyvanse 50mg for over two years now and feel very stable and happy on that dose. I recently ( in the last six months ) talked to my PCP about treating my anxiety, and he suggested we try Zoloft. I have noticed that my anxiety and depression have decreased in intensity and have allowed me to function better in my daily life. The reason I am concerned now about this recent medication addition is that I have noticed recently that I have gained a fairly significant amount of weight over the last four months (about 15-ish lbs) and I have a feeling that it is medication-related. I am at a loss of what to do now... Should I decrease Zoloft even though I have found that it is helpful for my mental health? Should I switch meds? Should I add meds? Should I focus on changing my diet and lifestyle instead? Thank you in advance for your thoughts.