So what's your question? How did you find out? Did she bring it up or did you stumble upon the information?
If you've stumbled upon the info, continue with your plans until she says something to you. Then, either include her in your plans (if there's room) or tell her that you've already got plans...maybe she can bring it up sooner next year. I guess this depends on your personality and your relationship with her. I could do it because people who know me expect me gently shoot straight. They know exactly what they're getting with me.
Quite frankly, it's unreasonable for her to expect you to adjust your plans at this late date. Maybe she's dealing with some feelings of being left out since the ex- will be there (no matter how well they get along). Maybe she assumes that she has automatic rights since you two (you and hubby) haven't stressed that you want to be alone. Either way, that's her issue, not yours. She's responsible for her own feelings and for what and how she communicates.
Be gentle, but people should understand that your birthday is YOUR annual time to spend how you want. What you want might change from year to year, but this time you're already clear about what you want.
If she is interested in having you guys over for dinner as a little token of her love, let her do it, but don't give in to a party if you're not up to it.