The law in Texas provides that it is a defense to a sexual assault charge (assuming that the sexual interlude was consentual) that the parties are within 3 years of age of each other. As little as 3 years and 1 day apart takes away that defense.
You should consult with a local lawyer in the event the parents do contact the police regarding charges. Many times when the behavior is consentual (that means your girlfriend confirms it was consentual), prosecutors do not really want to prosecute. If this is true, a lawyer may be able to put together a grand jury packet (with positive information about you) and obtain a no-bill (meaning the grand jury rejects charges.)
By the way, the age of consent is 17. When your daughter turns 17, then the law is that she can give consent to have sexual relations with another person of any age.
You need to put pen to paper and see exactly how far apart you are in age to gauge your exposure. As Cynthia says, if you are even 1 day over the 3 year mark then you are in trouble. So, if you are going to turn 20 before she turns 17 then I am guessing that you will have some exposure. If she turns 17 before you turn 20 then I believe my math is correct and you are within the 3 year rule.
As Cynthia also stated, when she turns 17 she can have consensual sex with any consenting adult. It should be remembered in today's society of camera phones and what not that it is still illegal to take pictures of her engaged in sex until she is 18.