My [28/f] MIL [52/f] has been feeling excluded since I got pregnant but won't talk about it

I'm pregnant with my first and am due in two weeks. This will be my mother-in-law's first grandchild.

As far as I know, she has always been a bit emotional, but otherwise rational. My husband and I never had any problems with her until I got pregnant. She is very happy about the baby, but is now making everything about her and uses everything we say and do in our lives as "proof" we hate her. I told her (adult) daughter about my pregnancy first, and then MIL was the second to be told. So, MIL thinks it's personal. When we buy things for the baby, if we don't immediately tell her, it's personal. So, I started a blog and took pictures of everything so no one would feel left out. She also receives texts about baby things. But to her, if we talk to any other family members, about anything, we must be sharing more with them and we must hate her.

My husband has tried to ask her why she feels so upset and left out, but then she will deny feeling that way. She will proceed to be cheery and act like everything is great. (She's generally cheery but will randomly interject with, "Everyone hates me" type of comments.)

My husband has recently received texts from his mom's wife, berating him for being so mean to his mother...but the texts don't actually say what's wrong. He also received texts from his sister along the lines of, "I don't know what's going on, and I'll always support you, but mom's heart is breaking."

What should my husband (or both of us) do? Could anyone make any guesses as to why my MIL is acting this way?

TL;DR: My mother-in-law is excited about her first grandchild, but since finding out I'm pregnant has acted like we suddenly hate her. We don't know why she feels that way and she refuses to talk about it.