Why is Botox suddenly causing my right eye to sag and right brow to drop? What suggestions/advice would you recommend?

As we age our forehead continues to sag and with it the eyebrows and upper eyelids lower. We start using our foreheads at rest to create an upper tension lifting up the eyebrow and eyelid complex.  I have seen many patients for many years and it is not unusual to no longer be able to treat horizontal forehead lines in my mid 30 year old patients because of this concern and can only treat their glabella regions which lessens the frown line "11" s  and also helps raise the forehead.If your doctor, only injected the glabellar region between the eyebrows and not anywhere on your forehead over the eyebrows, I can't explain why this is happening to you. One concern is if a doctor uses as a landmark the medial inner eyebrow and the patient plucks their eyebrows making the inner point wider than normal. The anatomic landmark to be sure to be over or to the inside when injecting the glabella to ensure the forehead doesn't droop, is the vertical line that intersects the medical canthal corner of the eyelids.The information provided in Dr. Shelton's answer is for educational purposes only and is not intended to constitute medical advice.  The information provided should not be relied upon as a substitute for consultations with a qualified health professional who may be familiar with your individual medical needs. 

thanks for your question 
It is either Rt eye ptosis or just too much botox or low injection site was done on the Rt side of your forehead 
Check with your injector 
If it is ptosis of Rt eyelid your injector can prescribe eye drops and it will solve the problem 
Good luck
Dr Lamia Wassef

If the injector has already touched it up with additional units, I would not recommend touching up until your next treatment is due (3-4 months). Although uncommon, more frequent treatments under the recommended time can lead to an antibody response and may make the Botox less effective. 
As far as the "droopy brow" goes, your treatment needs to be adjusted. This result happens when too much Botox is injected into the frontalis muscle or when it is injected too low. You can either discuss this with your treating injector and further adjust your treatment plan or see a different provider for an in person evaluation and second opinion. 
Best of luck. 

Hi and welcome to our forum!
Botox is a neurotoxin derived from clostridia botulinum bacteria. It acts to temporarily paralyze musculature in the area of injection, allowing overlying skin to relax, reducing fine lines and wrinkles.
If there is a natural weakness of the frontalis musculature on one side of your forehead, brow height disparity may be exaggerated with botox injection.
If injected too close to the orbit, it can cause temporary upper eyelid drooping due to involvement of the levator muscle.
Consider lower drug dosages on the side with brow ptosis at your next treatment session. Consider an alternative such as xeomin or dysport. Or perhaps, this may not be the treatment for you.
Best wishes...