J - look back at the 10/7 posts on my thread - I really think your BF is probably reacting the way my H does. After all - he realizes you're hurt, he's overwhelmed with guilt that not only did he say those terrible things to you about the flowers, and have the affair, but now he's messed up even more by not thinking about giving you the flowers (or maybe he thought about it when he bought them, then chickened out on giving them to you because they were so fraught with layers of history? Is he the kind of guy who would normally buy flowers for himself?).
Either way, when it comes up, he's frozen with guilt and fear that you will abandon him - so he clams up.
You might try addressing the question in an email - if that works better for you - simply stating why it bothered you and what you would like him to do.
"Dear BF - The reason I got upset aboput the flowers the other day was because I remember you telling me you wanted to be in a R where you felt like buying flowers. I took that two steps down the road and ASSumed it meant you still don't feel like buying ME flowers. I know that doesn't match up with the rest of your behaviors and words towards me, and that I'm reading a lot into this sitch that probably isn't there. What would really make me feel better would be if you would start buying me flowers (say, maybe once a week for a few weeks? ).
I know it may seem silly or forced but it would really mean a lot to me.
). 
