Without seeing a photo of you it is all a bit speculative, however, I will give my opinion based on your description.
First, is this condition only visible in photos? If so, you may just have an over-reaction to the flash and actually be blinking.
If the condition is present when you are examining yourself in the mirror, looking straight forward in a relaxed state, then you might have levator ptosis. This is a condition where the muscle (levator palpebrae muscle) that opens the eye has a poor attachment to the eyelid. This condition can be acquired with age or it can be congenital (born with it). At the age of 31 you likely would have a congenital version or partially congenital and partially acquired. Either way, the treatment is surgical and involves shortening and strengthening the attachment between the eyelid muscle and the eyelid itself. Depending on the details, this can be done through the skin on the outside of the eyelid or through the underside of the eyelid (conjunctiva). The operation is generally easy to recover from with little down-time and usually no pain. Success rates are very good with the most common problem being that the two eyes are slightly different afterward.