Reactivity is not advised, as years of reactivity between the two of you is what likely what caused the situation in the first place. If you are paying for her phone service there is no 'accidentally' reading a text; you have the right to review her behaviors in that way if you choose to do so, but only if that was the deal in the beginning when she got the phone. If that was not the deal then, you cannot change it now.
Teens move from a childhood understanding of morality (break the rule, you pay) into a more advanced morality (break the personal contract, you are the bad person). Use that fact of her development, but only if the deal was that you get to review her texts. She has broken a personal contract with you to keep herself safe and use the phone appropriately.
On the other hand, if reviewing her texts was not the original deal, you now know your mistake. To tell her about the text would be admitting that you invaded her privacy, which of course, breaks either an explicit or implied contract between the two of you. If you do this, you both lose in your relationship (trust).
You should also know that kids 'act as if' they are playing 'grown up' using all kinds of words, phrases, dialog that is pure fantasy. You can use what you discovered (without telling her) to open up an honest, clear communication with her about how she presents herself to others, and online or text safety.
Obviously, you should be stepping up your observations of her, knowing where she is at and checking to see if where she says she is she actually is. If she notices and complains about this, you can tell her that while you trust her, you know that young women her age get into situations sometimes that are risky, and it is the situations and other people she comes into contact with that you do not always trust.
The answer to your problem is not to be reactive, but to choose a response that keeps her safe and BUILDS your relationship. If you REACT, first of all you may be wrong in your assumptions, and secondly, you will harm your relationship with her.
The word 'discipline' comes from the word 'disciple'. A disciple follows and does what the older, wiser person says not out of fear of retaliation, but out of respect for the RELATIONSHIP...the disciple does not want to damage the relationship. And the wiser, older person knows that close relationship beats punishment every time.