I would invite her OUT for coffee. this way you are not showing up unannounced at her door trying to catch her off guard just to sneak a bit of time with the kid. you are acknowledging her as a parent and in control. Anyway...get her off for coffee, and tell her how you feel. That your heart is broken that you were not able to attend that party, and since then, you feel that maybe things are off. That if you have done/said anything that has been viewed or perceived as offensive, or rude that you want to apologize here and now, and lets sweep the floor clean. Give her time to speak. LISTEN to her. She may have taken a tiny thing and be blowing it way out of shape, but to her it may just be blistering a hole in her heart.

DONT show up UNannounced to try to sneak your way in for a visit. That will make things way worse! Please approach her with respect and love. EAT CROW if you have to, even if its not your helping to eat. I am sure you will do what you have to for the sake of the grandbaby.