I believe the ptosis of the right eyelid I had a forehead lift to correct, was caused by botox. Is this likely?

Possible. If you received a Botox injection near or into your upper eyelid. However it should have resolved in 3-4 months. Did you have your ptosis evaluated by an ophthalmologist?
Could be. If the ptosis of your right upper eyelid was corrected with the forehead lift, then the Botox could've caused relaxation of your forehead muscle which caused droop of your brow resulting in a "pseudo " ptosis . This is not a complication but a direct effect of Botox on the forehead muscles.
Depends on timing. Botox can cause ptosis of the upper eyelid in two ways: a direct effect of relaxing the muscles that raise the upper eyelid, and an indirect effect by relaxing the compensating muscles that raise the eye brow. This can last from 1 week to a few months after Botox injection. Once the Botox wears off however, the ptosis will resolve. It cannot cause permanent changes in muscle function.
Botox. No. Botox blocks muscle action. Droopiness of your eyelid is a potential side effect when it is injected near your eyelid. However, the effect is only temporary as the product only lasts a few months.