It is generally a comfort thing. given her age, it could be for a number of reasons that she has picked this up. She could have seen others doing it and be merely copying them, she may be feeling anxious and feeling the need to suck something, or she could be wanting the attention that doing it is causing you to give her. It is generally something they will grow out of, I would definitely try to discourage it, but wouldn’t be overly concerned. My son sucked his from a baby until around 5 at least.

Most of the time at this age it can be seen as a coping / comfort mechanism for stress, anxiety, exhaustion or boredom and is sometimes seen as an age regression. Quite often it is an temporary reaction to change (moving house, diffrent school or class, different teacher, a friend that moves etc) yet important enough to address due to the impact it can have on teeth.