K6 + Picopresso = bitter shot. Troubleshoot post

Just got my hands on the K6 and dialed in my first shot. My zero was at -4 and i used the setting at 45.

The shot I pulled had decent crema but left me with a bitter aftertaste.

I have used the same beans preground and the taste was far better though no crema.

I am using medium roast Bynemara estate beans. The beans were roasted on 27th December and were stored in the freezer.

What can I do? Where am I going wrong?