It sounds like she might have a chalazion. A chalazion is a painless little bump that grows on the eyelid. It will only grow until it gets hard and then it maintains the size that it got hard at. Technically it IS a little tumor but they are harmless unless they are in a portion of the eyelid that causes them to scrape over the cornea when she blinks. They are treated by slitting open the bump and removing the contents. It requires a general anesthesia and is a simple procedure.
The only way to tell if this is truly what it is would be to make an appointment with the vet to have her examined. Please keep us posted and let us know what the vet says.
Ghilly
It might be a stye...but sometimes dogs can get growths on the eyelid. These are often benign (ie not cancer), but can cause irritation to the eye and conjunctivitis.
If this IS a stye, then you could bathe it with warm sterilized saline solution, and see what happens. But if it's no better by next week, a visit to the vet would be the best idea.
To make the saline solution:
Boil some clean fresh water for 2-5 minutes. It might be best to boil it in a very very clean pan (no traces of detergents) rather than a kettle, as kettles can sometimes retain fragments of limescale which can be irritant to the eye.
Put this water into a very clean container or cup, add a teaspoon of salt, and dissolve completely. Let the mixture cool to warm-ish/lukewarm. Use cotton wool balls or pads to bathe the eye. Hold the salt water solution for a few moments on the eye. Every time you use a cotton wool pad, discard it, don't re-use the same one, that can spread germs into the eye.