Martin, I am so sorry that your grief has jumped back and hit you so hard and so deeply.
As others confirm, this is part of the grief process.
I don't believe you ever 'get over' losing someone you love, I think instead that you weave that loss into the fabric of your being, but that does take time, and it's not a one way street either with stops along the way to mark your progress.
The truth is, the street of grief is two ways, and you can meander slowly down it for a long long time, and then suddenly like today, you can speed right back to where it feels you started from, as if it all happened yesterday.
That hurts, it cuts deep and it hurts, but you will feel a little better tomorrow, and again the day after that, and the weaving process will go on, as it has since you lost your dad.
In time, the weaving will be ended.
You will always have days like this, when something prompts you to tell him something, but then you will smile, and think of him, and the pain you felt today will just be an ache that you have learned to live with, because in the end, we all learn to live with our losses.
We have to.
But it takes time, so allow your tears to flow, and think of your dad, and how much you miss him, and let the weaving carry on.
Feelings are what remind us we are alive, but they don't always tickle.