Quite possible that this could be a simple pimple.
Bacterial STD's do not really account for what she apparently described - except perhaps a Syphilis chancre sore, but Syphilis chancres are not painful, nor can they be 'popped' if I'm not mistaken.
Apart from that, If that's been there for months, then I sincerely doubt it could be Syphilis since the first stage of the disease is far shorter than that.
I would advise your sister not to mess with that pimple since its obviously not doing her any good when she does.
She could check her diet to see if there's anything in there that could be causing this problem firsthand.
And finally... you could instruct her to put on some lemon juice on that thing say twice in a day, or real aloe vera gel (from the actual plant) - also twice per day (both can have pretty good effects against pimples in general).
If you are worried about an STD, then I suggest you somehow scrounge up the money for a doctor's appointment for her so she can get tested just in case (but I doubt this is STD related).
She is a plus size girl but is training to get healthier.. could that have something to do with it?