I don't have a lot of ideas about what this means except maybe she is working hard to develop a physical skill - crawling? Pulling up? They say that sometimes they focus on one area and everything else kind of halts until they master whatever skill they are working on. I had the opposite issue with my daughter who is also a b/g twin. She focused on talking and put off walking until 18 months. She could say the word, "walk" before she could do it on her own.
That said, I absolutely think that you should share your observations with your pediatrician and see what he/she thinks.
Now, here is what I don't want you to do. Please don't feel guilty about how you parent them or think for a moment that some sort of unequal treatment by you has caused this. Being a twin parent is HARD! You are only one person, yet two little people need you and sometimes it seems like they got cheated because it seems only fair for every infant to have his/her own mommy! You spend a lot of time putting one twin in some device (swing, exersaucer...) so you can be free to pick up the other one who needs you more. The "squeaky wheel" does end up getting more of your attention. That used to break my heart, when I wanted to play with the twin who was happy in the particular moment, but I couldn't because I needed to comfort the one who was being fussy. At their birthday party, my daughter stuffed too much cake in her mouth and got really, really upset. I was torn between wanted to take her away to a quiet place where she could calm down, but I also really wanted to enjoy my son enjoying his first taste of cake. So I held her and stood in the doorway so I could still see him. I think that is a pretty good analogy for twin parenting - sometimes the best we can do is to "stand in the doorway." Things aren't always equal and needs aren't always met perfectly. But you do the best you can. If they are both attached to you and bonded with you, then you've succeeded. Hang in there, you do survive the first year! I am still working on surviving the second. Remeber that there are others out there who are "in the trenches" with you.