lump on dog side

I have a 5 month old boxer/german shepard mix that i rescued from a shelter 4 months ago.   She is her normal loving self, but i found a quarter sized lump on her right side that is raised and red but appears to have a scab over it.  I dont know what that is, she is up to date on all her shots and is scheduled to be spayed on the 18th, she isn't scratching at it, pawing or biting it and is acting normal, but it made me very worried.  I have heard i would need to have the vet do a biopsy, but does anyone have an idea what this could be? she is scheduled for an appointment in the afternoon today, but i cant stop worrying about my little girl! Can anyone help?

[jojo219]

Boxers are VERY prone to mast cell tumors, which is why you've been advised to have a biopsy done. The vet could also do a fine-needle aspiration of the bump to see if it is, indeed, a mast cell tumor, which would be less expensive than the biopsy. Then if the bump did turn out to be a mast cell tumor, you could just schedule her for surgery to have it removed. Best of luck to you... we have a boxer and love her very much and are dealing with the tumor situation ourselves right now.