Hi and welcome concerned. I'm sorry you and your girlfriend are dealing with a possible miscarriage. To be honest, my first thought was to check with a Dr, but if you'd prefer not to I don't think you can really confirm a miscarriage or not.

But, if her cycle is always very regular at 28 days, generally she would be ovulating around day 14. I don't know how long her period lasts, but if it's say 5 days, it started on the 6th. She would have ovulated around the 20th. So if you have had unprotected sex around that date, she could have fallen pregnant. To then start her cycle again only 7 days later, it could be a few things. How heavy is it? It could be implantation bleeding, but that's not usually heavy. I guess it could be a miscarriage but I'm no expert here. I wouldn't want to alarm you nor give you false hope.

So, I guess, if it really is very abnormal and she's not feeling well, I'd still advise to see a Dr. Good luck.