Mother of the year

Okay so I need some feedback on how I'm feeling and a gut-check on how much this is my raging hormones- please be kind in your answers.

I'm an only child expecting my first and only baby (after two miscarriages and plenty of challenges during this pregnancy as well) in January. My friends are throwing my baby shower in early December. I live in Maine, and my parents do too, but they spend winters in Florida.

Last winter my mother had a knee replaced and totally blew off rehab & PT so she still has issues with it- but she works a thriving real estate career and shops and is generally mobile. She also flew back to Florida three times this summer for a week at a time to catch up on business obligations so travel and money are not an obstacle.

So my mother tells me that she isn't planing to come to the shower- its an "inconvenient time for her business" and she's worried about her knee on icy roads. We have a tricky relationship (she's very self-centered and I'm sensitive to it even after all these years). I was fine with her not coming even though its been hard to explain to the other guests (and especially my in-laws who are super excited about this baby).

Yesterday she told me that she and my Dad have been very stressed so she booked them on a week long cruise during the exact same dates as my baby shower. Mind you, she wouldn't have to fly anywhere to take the trip, but that sounds like a lot of walking and time out of the country/without reliable communication "during her busy selling season".

I'm feeling an intense level of rejection and am even protective of this baby someday being disappointed like this by a grandmother with warped priorities and a lack of any family that cares. Am I just being sensitive or is she being super selfish? It's is normal for a mother to skip her only child's baby shower?