Why is the pain of mom's death worse now after 3 months?

Yes, it's normal. I got hit really hard after two years! The emotions of the last months (years?) of caregiving and decline were still with you strongly when your mom died. It was easier to accept that she no longer suffered. Now, you are remembering all of the things you have lost through time. The reality of being an orphan is hitting you, as well. These things take time to sink in.
If you find that you can't get over this grief fairly quickly, you may want to see a counselor just to talk it all through. Again - this is perfectly normal. I think maybe more so for caregivers.
Please do take care of yourself,
Carol

I think it's like when you're in a car accident. You are thrown so far out of your comfort zone, that all you can do is go on auto-pilot to take care of what has happened to your car etc. It's only later when you sit down and realize what, could, would, should have happened that it hits you. That's when the crying, shaking, anger, hits you with the reality of what just happened sit in. So I'd say give yourself a break, take the time to cry and mourn your loss, because it seemed normal when my mom died last year and the same thing happened to me.