ANY change in size, colour, shape or texture should prompt you to get it looked at by a GP, especially if these changes occur rapidly. If the colour or edges become uneven they are also not good signs. Also, as 4GS said, you must get it looked at promptly if it becomes itchy. If any 'satellite' moles marks or lumps should appear close to it then a mole needs immediate attention.
The mole itself doesn't necessarily need to be directly exposed to the sun to become cancerous or pre-cancerous.
Even if it should turn out to be quite normal you can still ask to have it removed if it annoys you. To back up what Abdulmajid says, it's a simple procedure that can be done at your GP's surgery and will only take a few minutes to sort out. Had it done myself on more than one occasion.
Where moles are concerned you should always think....If in doubt, check it out! Malignant melanoma is one of the most rapidly spreading and aggressive forms of cancer there is and early detection and removal is of paramount importance.
I'm sorry if I appear to sound a bit melodramatic but I lost my dad to metastatic malignant melanoma. I'd rather scare someone into getting a mole checked than think they might not take any action at all. Stuff feeling like a hypochondriac....go and get it looked at or removed! NEVER take any chances with moles that concern you.