ps - Keep in mind, when a doctor sits a woman down and says "You are 8 weeks pregnant," he is not saying "You conceived 8 weeks ago." Pregnancy is counted from the first day of the last menstrual period, by doctors, presumably because that is a big, obvious date, easier to track. "8 weeks pregnant" in medical language means about 6 weeks from conception.
myenzoorka has it right -- the dates could add up, if she got pregnant by you in February. If she is pregnant, and if by you, telling you she is pregnant with twins almost has to mean February is the date, unless she simply extrapolated twins from her hCG level. (Her hCG level in early April wouldn't really be enough to tell her she had twins if she got pregnant in late March, but sometimes people do extrapolate from a high number that it might be twins, and she might have heard the doctor as saying that even if he made a more qualified statement.) As for her not telling you in March that she had missed a period, well, plenty of women miss a period and wait to see why without making an announcement to their boyfriend. (She might have been simply wishing it wasn't true.)
So, as the other ladies say, don't blow her off as a liar about being pregnant, and take a DNA test because you need to be ruled out or in as the possible father. The dates add up enough to take a test.
High HCG levels mean you are pregnant. You say the math doesn't add up. What figures do you have? Do you know her due date? How far along she is? Did she give you the date of her LMP? Or even a possible date of conception? With the numbers you gave, the time period can add up. Let's look at the numbers. Since you weren't specific with dates, I'll create approximate dates.
Let's say conception happened the first time - say, February 28th. Let's say her last period started on February 14th. By April 9th, she would be just a couple days shy of 8 weeks along. An ultrasound can find twins at 8 weeks along in pregnancy, so that is a possibility.
If, however, conception happened March 26th, it would have been incredibly unlikely for her to know she was having twins - unless, of course, she just assumed that from her hcg levels (some people assume higher hcg levels automatically means twins, but that's not always the case). But, her doctor may have told her that she's having twins because of the levels - and I'm sure a doctor would check for the other causes of hcg before telling her she's having twins.
Just because she didn't tell you her period was late, doesn't mean she's not pregnant. If you were able to see her records stating her hcg levels were high... she's pregnant. If she's lying, well, you're off the hook, so to speak. BUT, I would recommend getting in contact with her as your argument that she's not pregnant doesn't sound too solid to me.
Well as far as I know HCG is only produced when you are pregnant or taking fertility drugs so she is most likely pregnant. If you are unsure about the dates not matching then please consider a DNA test. You shouldn't have to be responsible for a child that isn't yours but if it is then you should be.