Increased size or an increasingly irregular appearance, especially at the edges, always warrants medical attention.
This isn't a 'sure' sign for cancerous transformation of the regular mole, but, must be checked by your dermatologist.

Hi Fedilia, there are 5 important points that usually physicians and dermatologists suggest to be aware of:

A Asymmetry
B Border
C Color: Different colors in the same Mole (red/white blue)
D Diameter: If its too big
E Evolution (Change in form and size)

As mentioned by MandMs, there is not an exact symptom that may indicate that something is wrong, but any of this 5 points are enough to visit an specialist.

Hope this is useful for you.