Hate to say it, but it is at least possible she is not telling you the true story. I guess one thing you could do is go have a semen analysis and see if you are capable of fathering kids. If not, then she has some 'splainin' to do whether she really was pregnant, or no.
We occasionally do hear on this site of someone whose girlfriend has made the big pronouncement, either of "pregnancy" followed by a "miscarriage" when pressed to prove the pregnancy, or just of a "miscarriage" after the fact that could never be proven.
If she is saying you are the father, she owes you access to the doctor who would be able to tell you the details. If she won't let you have such access after dropping a bombshell like that, I would watch my step in the relationship. Some women are a whole lot of trouble and there are a lot of nice women out there who are not a whole lot of trouble.
If she lets you talk to her doctor and you are convinced she did indeed have a miscarriage, then DO get that sperm test. If it shows you could be a dad, then all is well except you'll have to do a little bit of patching-up in the trust department. if it shows you couldn't be a dad, well, then, it does sound like a little bit of explaining is in order on her part.
Good luck. Do cut her a little slack ... men can't lie about being pregnant, but they often do lie about paternity, so probably one sex doesn't really have it over the other in terms of mean tricks relating to pregnancy. I hope she stops stonewalling you and that if you find she was telling the truth about the miscarriage, you also learn you can father kids.
Wow!!! Well first let me say, you do have the right to feel the way you do. Alot of times, we have the answers to our very own questions.Now when having a miscarriage, its going to be a pretty darn heavy flow, cramping and it usually last more than one day, or overnight as she told you. I just had my first miscarriage on the 4th of June. I have three children already from a previous marriage. I'm a newlywed for a year now, and my husband and I have been trying to have one on our own since this past March. Anyways, I had what they called a chemical pregnancy. I received a positive HPT (home pregnancy test) on the 2nd and 3rd of June, I took another test (that's what us women do..like watching the lines get darker means more oregnancy hormone in your body) and the test on Friday came back negative. I was like what the heck! So I went to doctor off ice, which revealed a negative blood test, and that hcg levels were low. They sent me home and said I was having a miscarriage. Later that night, I started to bleed and bleed heavy..it went on for nearly three days. So I hope that insight helps you. Had to go back to doctor to make sure my HCG levels had returned back to 0.