Botox injected in one muscle groups can cause another muscle group to attempt to compensate. However, the effects would not be noticed 21 months later. The festoons that you are describing are most likely the effect of loose skin that may need to be surgically treated to improve the cosmetic results. I would consult with a board certified cosmetic physician for evaluation.
Festoons can occur on the lower eyelids. Several factors are responsible for this namely Volume loss, skin laxity, muscle laxity, gravity and time. Botox might make it better but its possible you might need surgery to tighten the muscles and then do other less invasive procedures to maintain the results of the surgery
None of the changes you see now are likely due to botox
injected 21 months ago...the botox is gone...now you need to see your doctor and decide what can be done about your current issue...
Muscles can try to compensate for those treated with Botox
Yes, if you have had Botox you can have other muscles that are working harder to try to compensate for that loss. However, you should not work them hard enough to be noticeably stronger or different and all of the effect will be gone at 21 months.
Recruitment of muscles not treated by Botox
It is common to see recruitment of muscles unaffected by Botox to account for movement around the eyes. Your issues should be evaluated and discussed with a board certified dermatologist.