That's not a random act of kindness that's I want this treat so go get me it.
Is that how that conversation happened, in your head? Funny because I thought OP asked her DP if he could get her lemon water because she wanted her emotional fix during that time of the month, and he categorically said no.
In my world, if I ask for something at an inconvenient time, anybody including my H has the right to say they don't want to go. I'd understand that, and of course I am not going to combust into flames if I don't get my cream soda. Depending on my mood (yeah, periods, hormonal, mood swings, it happens, get over it) I'd either have distracted myself or walked to the shop myself. And yes, there would be the slight, yes, but also inevitable disappointment that he did not care enough to walk a few steps to alleviate my pain. Nobody can tell me that I should not feel this way, especially when I would have done so to him in a hearbeat, and he knows it.
But if and when said family member - let's say SO, since it is a romantic partnership in this thread - does take the time to do it for me, well, then, that's the kind of stuff most of us live for and expect from our loved ones. I am not going to be ashamed for that. Neither should the OP be.