You're definitely not being dramatic or oversensitive about this. You're meant to feel like you can be open about sexual things within a relationship without judgement. To open about something you'd be willing to try (with her prompting, no less) and have her act in such an awful way, of course that's going to be hurtful.

You have no reason to feel embarrassed and ashamed. There's nothing wrong with what you wanted to try. I imagine a lot of guys are interested in anal play (as in, being on the receiving end), it just isn't talked about as much due to the (wrongful) stigma. There's everything wrong with how she reacted.

As others have said, it may be she was trying to do something she thought you wouldn't be comfortable with so the blame could fall on you for turning down sex, not her. While this is manipulative and not okay, it's important to consider why she may have done this. Have you discussed the reason behind the lack of sex? Is it just a lack of time or is it possible she's feeling more insecure or experiencing a lack of libido post-partum? Perhaps she feels guilt over the lack of sex and thought you turning down sex as opposed to her would help.

Another thing that occured to me, although a longshot, is she may believe you're lacking interest in her and exploring reasons why. While not true, a lot of people tend to automatically equate a man being into anal play as being gay; this could have been her way of testing that, which would explain why she had such a bizarre overreaction after being the one to ask about it.

Her behaviour isn't very logical, but from what I've heard, post-partum can be an extremely hard time for mothers. Talk to her about it. Explain the situation from your perspective, how it hurt you and how and why her behaviour wasn't okay. Hopefully you'll be able to get more clarity on the situation and work out a way to move forward, although I think it'd be completely understandable if you feel a bit more guarded when it comes to sex with her from now on.