When she was previously pregnant, was it confirmed by ultrasound, and if so, when was the last point in the pregnancy that it was confirmed by a doctor? (Is this confirmation something you know, i.e., you were with her, or just something she told you?)
I ask because even if she miscarried as early as 3 months along, she would know she had miscarried and it would not be a surprise now to discover that the old pregnancy was gone. We do get tales on this website of a gal telling her boyfriend she is pregnant when she is not, either because she is genuinely confused, or sometimes in order to keep him around while trying to get pregnant again.
A scanner would not confuse a 7 month and a 7 week baby. With a 7-month pregnancy your girlfriend's stomach would be visibly pregnant even if not huge, so somewhere earlier than that, she was not pregnant even if she started out pregnant months ago.
Anytime a baby is delivered at or after 20 weeks and isn't a live birth the loss of the child is called a still birth/born not a miscarriage. If she had a still birth/born after 7 months it would take a least 6-8 weeks for her body to get back to normal just as it would if she had carried the baby full term. Now am I saying what you've described couldn't very well happen-no. What it sounds like to me that some your information just a little off. At 7 months most woman are showing some form of their pregnancy in their appearance but at 7 weeks the baby is just like the size of a rice grain, so if you haven't been preggers before you more than likely will not be showing. Please tell your girlfriend to express this matter and or any other concerns she may be having with her OB/GYN ASAP. Best wishes and God bless as well!