Where did you have your mammogram done, was it at one of the breast screening units? If so a letter would come from them not your GP.
You really need to pluck up the courage to call your GP to find out what it is all about for your own peace of mind.
Even if it is a recall from your first mammogram, it does not mean that it is cancer, quite often there can be something as simple as the mammogram is not clear and they want to do it again, also there a many benign breast conditions.
Went for a general Mammogram on the 12th April,on 18th I had a miss call from my GP then a urgent letter from her to get in touch, I'm quite Nerves as that's the only reason my doctor would need to get back in regarding my Mammogram.