It does not sound very likely, if she has taken tests and they are negative (and they were done to proper protocol as in: first morning urine, the test strip was immersed in urine for the right amount of time) then I would say you two can put your mind at rest. You did also say that her periods are irregular, so her being late shouldn't have been too much of a big things. Womens bodies work in mysterious ways, we are never 100% the same each cycle which is why she may be cramping more right now. This can be due to stressing about being pregnant, not to mention a whole variety of other factors like medication, illicit drugs, mood, your diet, your sleeping patters... Also every couple of months women will get a period that is typically more heavier and painful than the rest. This is our bodies kind of doing a flush out. Tell her not to worry, but if any of this does not set your minds at ease, then I'm sorry but she will have to go to the doctor. This is the only deffinate way you will find out. So it's either she ***** up her pride and goes to the doctors office, or she accepts the negative tests and you continue on with your lives. Just remember, no one on here can magically see into her uterus, so we cannot just come out and tell you the answer. If you really think she is pregnant, do the right thing and go to the doctor.
If the time that has elapsed between the unprotected sex and the last test she took is over two or three weeks, and if she took the test with concentrated, first-morning urine, you can rely on the results.