If it's a persistent problem and not just happening when your rabbit is feeling stressed, it could possibly be a prolapsed gland if it's bulging out significantly and constantly. Or if it seems worse when your rabbits head is lowered, your rabbit seems to always be trying to rest with her head and upper torso elevated, if you're noticing any increased or labored breathing, or unusual lethargy, this can indicate a possible heart issue or thymoma. Though thymoma in such a young rabbit would be unlikely. But a heart issue is definitely something that should be checked out as this can be a common ailment in large breed rabbits like flemish giants, and consistently protruding third eyelids can occur with heart issues.
https://rabbitwelfare.co.uk/rabbit-care-advice/maintenance-handling/caring-giant-rabbits/
Medirabbit: protruding third eyelid
If your vet isn't a really experienced rabbit vet, I would suggest finding one if possible, especially if there's a chance it's a heart issue.
https://rabbit.org/vet-listings/