Unfortunately no one on this forum is able to tell you for sure whether it's skin cancer or not. Only a doctor's appointment can give you that reassurance. I would try to get a sooner appointment if it's worrying you (sometimes ringing early in the morning and seeing if any cancellations come up).

This could be anything from a bone spur, calcified insect bite, benign growth - skin cancer is only a small possible out of many options. Don't diagnose yourself or worry about something that isn't confirmed.

Best of luck with your trip to the doctors - you'll hopefully be given some good news and this will hopefully not even be in your mind come August!