It depends on what your current credibility is like with your girlfriend presently. Have you already been caught cheating, has she suspected you of cheating openly. You see whatever your approach is to this problem is going to depend on whether you are already seen as a two-timer in her eyes and if that is the case your relationship was already rocky before this mishap happened. If your relationship has been quite solid and she zero reason to suspect you may have been two-timing her.
Do not try to internalize what you think she may thinking, you know nothing at this point! SO SAY NOTHING! BE NATURAL!
She may have not even received the text message.
Here is what I think you should do as juvenile as I think it comes across. ( others share your feedback )
First of all do not call her and discuss anything, If she happens to contact you. DO NOT BRING IT UP! As far as you are concern you did not send her a text and you know nothing, life is the same, there has been no change.
If you have not had contact with her yet, do not initiate
Go to Petro-Canada purchase a hobby phone for cheap ( I don`t have one ) but clearly YOU should have one. I heard they are affordable, activate it and send a series of random texts to her phone, I will leave you to decide the content you want to text to her, in addition send some to your current phone theses ones need to be similar in content to one you originally you sent her about ( Jasmine at Eros ) I do not think I should remind you but try to be vague even in those texts to you own phone and hers, the idea is appear as if you have been getting odd texts as well, not expose information about the hobby. Pick names of SP you doubt exist and cannot be searched out on the internet to link back to you perhaps throw a few homosexual ones in there ( come to think of it send her a homosexual one too ) just send some random texts, for example, send stuff that indicate it was intend for another person. The whole idea I am trying to manufacture here is that you, her and whoever else out there in the world are receiving random text, some clean, some border line erotic which you are victims of in addition to yourself.
Because of the content and the embarrassing content she likely will not call the provider, but might ask you to call them to investigate, at which point you can make that decision if you should or shouldn`t it depends on how much you need to convince her still or has she taken the bait and believes whats going on with the phones and does not want to waste time or having an embarrassing conversation with the provider.
AGAIN if she does not raise the topic DO NOT initiate the discussion, but be prepared to by putting all the things in place above ( if you decide to take the advice ) so you have an escape route. If she asks why you did not bring it up, simply say that you have occasionally receive unusual texts in the past, you called to complain but the cell phone provider stresses they can do nothing about unless you are being stalked or a crime is being broken and which point they can present it to the cops
Word of advice if you get out of this one with zero damage. Make use of a hobby phone, some of us need them some of those do not.
Cheers and good luck!