I feel so upset and really stupid but don't know if I'm overreacting. 
Just had a huge argument with DH because a woman has been texting him again. This is a supposedly joint friend, however DH has just informed me that she is actually not my friend at all 

This woman has a habit of texting DH. DH says that it's just quick snippets of information, nothing long, but he refuses to show me the messages, even though he knows how upset I am about it. He said he wouldn't show me messages from anyone, not just her.
I feel so hurt and used. To he honest I feel like I've been made a fool of by both of them. DH has pushed me into being friends with her, I've invited her on days out, looked after her children having her so much in childcare over the years.
But then whenever there's something happening or she wants to know something, she will always message DH. It's honestly not jealousy, it's just more a case of why would she do that?
I've told DH that's fine if he doesn't want to show me the messages but to have some self-respect, I won't be having anything to do with her anymore. No more being overly kind, offering her my children's old stuff, childcare, nothing.
Am I over-reacting? I just feel so sad and mad at both of them 