The winger appeared to be fouled by Black Cats counterpart Aiden McGeady in enemy territory but the decision wasn't given by the referee.
The visitors then went up the other end to double their advantage when Jordan Jones profited from a Sean Raggett error to finish over onrushing keeper Craig MacGillivray in the 58th minute.
Charlie Wyke had opened the scoring for Sunderland on 16 minutes.
However, Williams reckons Jones should never have found himself in a position to score – with McGeady even admitting it was a foul.
He said: ‘The referee said to me it was shoulder to shoulder - but he (McGeady) didn't get the ball?
‘I've knocked it in front of him, the ball is over there and he has pushed me to the floor. He even said it himself it was a foul.
‘The ref said there was a throw-in, but I said we wouldn't be down that end of the pitch if there was a free-kick would we?
‘The part from that incident to the goal isn't good enough, though, and we all know that
‘It's not a hard decision, that's the most upsetting part. There is a chance there but when it goes to 2-0, it becomes incredibly hard - especially without our fans.’
Even despite feeling Pompey didn't get the rub of the green when it came to key decisions, Williams didn't think the hosts did enough to win the game.
Jackett's side have now lost five of their past seven league games as their promotion bid continues to unravel.
Williams added: ‘We could have had a penalty for John (Marquis) in the first half then that happened.
‘We didn't create enough chances and couldn't break them down.
‘As a team, we have also got to defend better.’