What Causes A Developing Dark Spot On The Big Toe ?

Hello. I am an 18 year old female and I have just recently noticed that a dark brown spot on my right big toe (on the actual toe/beside the toenail not underneath) that seems to be getting bigger. I ve had this mark on my toe for as long as I can remember. It started off as a little dot and about a few months ago I noticed it looks as if it is spreading into a smudge.I am planning to go to the doctor in a week for a checkup and I was wondering if I should be concerned about something serious such as skin cancer. In the meantime I am worrying like crazy and I m just looking for some kind of relief- good or bad. I hope you can help me.Thank you