It sounds like you need to caulk around the window sill in the inside where you are feeling the draft.
As stated above, caulk but I would also check about caulking around the outside as well.
Given the window size, you may need additional insulation via heavy draperies for this window. If space around window is not well insulated, heavily lined/thermally lined floor to ceiling draperies may be considered.
Yes! I paid through the nose until I added thermal drapes. I open the drapes in the day then close them at sundown. My heating and AC dropped dramatically.
I agree with caulk as a temporary fix. Locate where the draft is and caulk. When the weather warms up you should look into the wall space and window frame for adequate insulation. You can use door and window spray foam if needed.