You have asked a complicated question. This question actually has three components - (a) what state's law applies? (b) what is child pornography? and (c) what other offenses could the conduct you described relate to? This involves the interpretation of Illinois criminal statutes and cases about the statute.
First, the law that is applicable is that of the state that can enforce said law. I know that is a mouthful, but it is important. Person A is in Illinois, and person B is in another state. Most of the time child pornography laws are similar but not always the same. It is likely that both states could arguably enforce their laws against you, but for this answer's purpose I will only comment on Illinois law. Keep in mind that the laws where your girlfriend live will likely be different and may be as applicable as Illinois law.
Second, Illinois defines "child pornography" in its criminal code at 720 ILCS 5/11-20.1(a) (2009). You can find the statute in its entirely on the Illinois General Assembly's website at www.ilga.gov. In a nutshell, child pornography in Illinois requires the recording, transmission, or possession of children "(i) actually or by simulation engaged in any act of sexual penetration or sexual conduct with any person or animal; or (ii) actually or by simulation engaged in any act of sexual penetration or sexual conduct involving the sex organs of the child or severely or profoundly mentally retarded person and the mouth, anus, or sex organs of another person or animal; or which involves the mouth, anus or sex organs of the child or severely or profoundly mentally retarded person and the sex organs of another person or animal; or (iii) actually or by simulation engaged in any act of masturbation; or (iv) actually or by simulation portrayed as being the object of, or otherwise engaged in, any act of lewd fondling, touching, or caressing involving another person or animal; or (v) actually or by simulation engaged in any act of excretion or urination within a sexual context; or (vi) actually or by simulation portrayed or depicted as bound, fettered, or subject to sadistic, masochistic, or sadomasochistic abuse in any sexual context; or (vii) depicted or portrayed in any pose, posture or setting involving a lewd exhibition of the unclothed or transparently clothed genitals, pubic area, buttocks, or, if such person is female, a fully or partially developed breast of the child or other person."
There are three key ideas here -- one is that the image or information must pertain to a person under the age of 18 or is severely mentally handicapped. The second is that the content must be "lewd," which cases interpreting the statute define by a set of factors looking at the setting, clothing, pose, overall visual depiction, and intended sexual response from the viewer. (People v. Lamborn, 185 Ill.2d 585 (1999)). The third is that the child pornography statute only deals with "visual mediums," so presumably words and text messages do not apply. This leads to the third issue.
Third, you ask about child pronography but it is likely other criminal statutes in Illinois may be applicable. Child pornography only deals with visual medium. Other crimes such as "Indecent Solicitation of a Child" and "Sexual Exploitation of a Child" may come into play regarding text messages and words. The definitions and statutory language for these and similar crimes can also be found on the ILGA website.
Unfortunately there is no "bright line" area in criminal law. You can only do the best with what the statute says. I would recommend doing what lawyers do -- read the statutes and make an educated evaluation based on the language and what you know of the facts.
NOTE: This answer is not intended to be legal advice and should not be construed in that way. This answer does not create an attorney-client relationship and no such relationship may be created absent a signed retainer agreement. Licenced in IL.