Because she sees you as the gateway to the grandchildren, and as the new woman who is (at the moment) the challenge to her title of Family Matriarch.
So she wants to keep on seeing you so that she can carry on undermining/being rude etc. - letting you know that you are being Put In Your Place.
This is all so that over time, and certainly when babies arrive, she gets to carry on being controlling and bossy and gets to say what happens in the family. And hopefully gets to take over a bit with the grandkids.
That's the idea with the lamp - totally ridiculous situation, who gives a fig where your lamp goes - nobody, not her either - but what she does want is to be able to tell you what to do.
So yes you need to take notice of this and act on it. Either:
a. Go, see her, and carry on sticking up for yourself (and crucially, make your DH stand up for you too) - with a smile, let her know that she has met her match and if she wants happy grandma time, she needs to at least offer YOU and your family basic politeness;
b. Don't go, make it clear it's because you think she's rude - different approach, same message - be nice to me, have some BLOODY manners, or forget it.
Option C is what a lot of DILs choose - try and keep the peace and spend years in a pit or resentment at getting treated like shit and having their own decisions on their own homes and families overruled.
Disclaimer: No MIL-hater here- simply going on what the OP has said - sounds like this is one rude, pushy family member who has a bit of an agenda here and wants to portray herself as someone who gets to be rude to OP with no comeback. Knock that on the head, I say!
You started well with the lamp...