Well, I can certainly understand why you feel uncomfortable about this. It does sound like she's trying to meddle, but then again, who knows what he's been saying to her? Flirting w/ someone can sometimes send mixed signals; maybe she thinks he's interested in her?
From your description of the situation, he's lied to you about keeping in contact with her after he said he wouldn't; not being able to trust him should be a huge warning sign to you... A man who values his relationship with his girlfriend, doesn't stay in contact with another woman who sends him provocative photo's and flirty messages, especially if he knows that it makes his girlfriend uncomfortable... If you've had an honest conversation w/him about how this makes you feel and he continues to be dismissive towards you, then you should consider moving on with your life, without him. I wish you the best :)
Oh my. Would ruffle my feathers too. I would not want her to be 'around' in any way. But the real problem is your boyfriend here. Why is he keeping in contact? What is he getting out of it? He knows it bothers you but that matters less than his contact with her. On some level, this contact with her feels good to him. That would be a deal breaker for me personally. And I'd tell him that. He's sending you mixed messages by being with you while carrying on this 'friendship' that is troubling to you.
My husband had female friends when we started dating and several that I had wondered if they didn't want more from him. As we got more serious, all of those people were weeded out by my husband. The dynamic between them changed when he was committed to me--- and he didn't want any problems. So, those 'friends' disappeared. Some of his female friends are still friends but they are now 'family friends'.
So, have a heart to heart with your boyfriend and express this to him. If he is not going to agree to put you and your feelings first, then he does not fit the bill to be your boyfriend. You want someone committed to you and trustworthy. He either will be or I'd move on. good luck and let us know how it goes.
I don't think you're overreacting. It's possible his relationship with her means nothing, but if that's the case he shouldn't have such an issue with being honest. It's also very possible that he's saying it's nothing because for him it is nothing, but he likes the attention she's giving him. Still not a good enough reason for him to continue to text her. It may not be intentional, but for him to tell you that you're being controlling is actually very manipulative. It's a way for him to get away with doing what he wants and simultaneously turning you into the bad guy for even doubting him, when he's actually done very little to earn your trust. I think your reaction is very natural. And it's the same one I would have. So keep putting your foot down. You should be completely comfortable voicing your discomfort in your relationship and if he's got any respect for you, he'll wise up and put an end to the flirtation.
I'd try to figure out why you're with this guy, he does not sound very committed if he is letting his head be turned by this girl, which it sounds like he is. You are worth a LOT more than that. Get busy, and find something in your life that is rewarding that is not him. It will help you clear your head about how good you can feel through your own capability and competency, and that, I promise, will help you look at him through clear eyes.