you're right people should not use Barcelona match against Rayo vallecano to conclude or judge the strength and performance of this Barcelona team because some times in football these things happen you don't expect Barcelona to win all the games in a season, there are some that will be lost and even draw what is most important is that after a bad result a team can get back to winning ways again.
I'll list the three teams Barcelona played against in those first matches and their results:
First match:Mallorca 0-3 Barcelona (easy win for Barcelona against a weak opponent; note that Barcelona didn't concede a goal in this match)
Second match:Levante 2 - 3 Barcelona (difficult win for Barcelona against a weak opponent; note that Barcelona conceded two goals in this match and were very close to losing, but scored three more goals)
Third match:Rayo Vallecano 1-1 Barcelona (draw against a weak opponent; note that Barcelona conceded one goal and only scored one goal in this match; they failed to score three goals)
When you look at Barcelona's performance in the last 3 games against weak opponents, you can see that they are declining with each game, which means there is a high risk that they will constantly lose points in the next games. In the first two games, they managed to score 3 goals, but in the third game, they only managed to score 1 goal. Their defense was good in the first game, but in the second and third, they declined.