Low LP gain

Hello,

So recently I had a good winning streak playing Udyr ( 12 wins, 3 of them I got carried hard ^^ ), but for some reason before reaching my promos to Silver 2, I have been winning less LP.

Before my winning streak I've been getting like 20 or 21 LP, but now I only get 18 or 19.

Is this normal or a bug or something ? I am not complaining just wanna know.