Does this happen only when you've had oral sex w/ your girlfriend? If so, then it might be due to spermicidal lubricants or contraceptives. If not, and it happens frequently (aside from sex), then you should seek advice from your doctor...
Have you asked her if she has changed her birth control lately? If this continues, you should consider telling her that oral sex has become unpleasant because of the taste and numbness you're experiencing. Having an honest conversation with her, might help answer some of the questions you have.
She might have used some anesthetic agent to feel less pain. This could give you anesthetic effect on your lips.This is just one possibility. There may be some other causes as well.If numbness continues to be a problem, you might have damaged some nerve.
This only happened once and she is not or has ever been on birth control. I just need to know if there is any other possibility before talking to her about it.
Local anesthetic's, diabetes, hypoglycemia or low blood sugar can cause numbness and an odd taste in your mouth also... Unless you have more evidence or proof that she is cheating on you, you would be left with only your gut instinct to fall back on. Personally, I wouldn't assume infidelity is the issue unless I have concrete evidence to present to my partner. Meanwhile, you might consider being tested for any STD's, just to be on the safe side.
In my opinion, no relationship is worth the risk of future health problems. It does sound like you doubt her honesty and integrity at this point, which can cause major problems in your relationship whether cheating is the issue or not. If there's no trust in your relationship, it will fail. I suggest speaking with her openly; tell her that you feel that something is causing a rift in your relationship and that you would like to resolve it. Ask her if she feels the same way? Tell her that you want complete honesty in your relationship. Keep in mind, unless you have hard evidence, it's not wise to accuse anyone of cheating...
Best of Luck to You