That looks identical to some OG Platinum that I have still in veg. (About the same size.)
And yes, when you switch over to Flowering, the vertical growth will slow. Perfectly normal. JealousGreen makes a good point below I think regarding lighting.
Farmer Tom uses metal halides during flowering to keep the stem growth as tight as possible. Blue light is great for keeping the plants from stretching out during flowering.
Metal Halides also put off (Of course) more Ultra Violet Radiation. The more UV the plant receives, the more THC it will produce.
Good Luck!