My GF and my friend's GF (both from poorer 3rd world countries) traveling to see us in the US had very similar yet different experiences based on one primary difference:
My girlfriend is employed. So once they found that out, they had little concern and it became much more relaxed. My friend's girlfriend is not (student, finished undergrad and taking a bit of time off before starting a PhD program). They give her the third degree.
It doesn't help that it can be pretty arbitrary depending on which agent you get, too. Having a similar description to yours, customs agents have always let me walk through without difficulty. Except for one time coming back from Canada.
