I can appreciate that this is a worrying time for you, however things have changed regarding breast treatments in recent years and so canceling your mammogram was not a great idea.
Do ring up and rebook it this week. Many women have lumpy breasts due to glandular tissue or 'fibrocystic lumps' (which are entirely benign.
If this brown spot is solid then it may not be related to the breast, it may be a type of wart however it could also be something less simple and need treatment.
As with anything with breasts, the sooner treatment is started then the much better the outcome. If this is still the size of a peanut then it is time to get it sorted now. It may simply be a sebaceous cyst.
If you wished to take a close up photo, I may be able to give you some more options however it really does need to be examined in person.
If you are worried about seeing your doctor, perhaps you could take a friend with you?
A flat spot can still be a wart, especially if it is scaly. But if it is growing fast then even more reason to have it treated quickly. If you felt the area affected was superficial rather than deep then you could be seen by a dermatologist rather than a breast surgeon.