Hey Black Cat, I'm not ENFX but as an ENFP who has had and has close friendships and dating relationships with INXP - I'll say diplomatically that you are not able to see eye to eye. But that labelling someone else's emotions as 'irrational' - especially from an INFP - to me seems ironic and particularly unproductive in a personal relationship that will never change (you will always be your mother's son). I have found through experience that sometimes you INXPs can be really stubborn and stuck on being correct and unable to really put yourself in someone's shoes, even with your Ne.
Emotions don't have to make sense in themselves, they always lead you in the right direction back to intent like you say, though.
She basically feels like she put herself out there for you in buying you the flowers and your response - while you don't characterize it as 'rejection' - IS a rejection for her. Because you obviously felt uncomfortable or baffled by it and you yourself say that you didn't want it - it made you uncomfortable because you saw it as an obligation or something else she could hang over you.
You didn't "get" it immediately and accept immediately simply a sign that she is going a little above and beyond for you - because loving someone means going above in beyond in tangible object-specific demonstrative ways and she loves you.
It was definitely symbolic of you rejecting her or rebuffing her show of love.
Basically, she is gonna think you didn't even "care" enough to recognize what it means for her to do this for her, and in a way, she's right
I guess I'm in the minority here because you sound a little cold and closed-off to really relating to your mom at least even in your OP. I'm not judging, I'm sure there are many other stories and really a long history of misunderstandings and frustrations that bring you to this point (and I know that Fi, once it feels threatened or under attack will retreat and hold its ground)
Just wanted to point out that you even broke out type personality theory and the love-style theory - if you were to look at this totally unbiased and 'rationally' - all you'd have to do at this point is plug in "A is to B, and when in C situation, do D" and it's very clear cut what her actions mean/what she wants/the appropriate response in the situation.
But, you say you still "don't get it". I don't think it's so much a matter of "not getting it" as "not approving of it" and "not wanting to bend to her way" because it's frustrating for you and it makes you angry that she won't or can't understand your POV and you find all this other stuff totally unecessary.
Why can't you make that extra step, especially if it's not really a "big deal" and it'll mean so much to your mom?
I think perhaps this is exactly what is going through your mom's mind in these moments and I think there's some truth to it.
We inconvenience and challenge ourselves all the time to reach and comfort people that we love, for no other reason than we love them. Now, if that's not irrational, I don't know what is!
PS I don't relate similarly to my parents either - they are both hardcore NTs, dad is INTJ mom is INTP. However, I am significantly older than you (been out of the house for years! Woot!) and have the luxury of hindsight and indepenence to help me. I know how my parents show their love and I know how to show my appreciation for it in a way that they understand.
My own POV for this is just to speak in a way that they can understand and then take care of myself on my own. You may not need your mom's demonstrations of love, but she does from you. I also am horrible at remembering dates and cards and all that, but if something is important to a friend or someone I'm dating (and I care enough about the relationship) I will make an effort if for no other reason than to please them.