Is it preflowering?

Photos are not going into flowering with a 18/6 , a pre floxer is the stretch aka the moment when the plant is getting taller in begening of flowering. Your plant is showing her sex , it happens before flowering and you'll only get a fex pistols like this. 12/12 is inducing flowering for a photo

Something doesn't seem/sound right with this whole situation.... Are you sure your on 18-6.. 18 hours on and 6 off?... Because your plant actually looks closer to a 12-12 from seed. It doesn't have that bushy look like it should from being in Veg for 4-5 weeks.. Your plant looks healthy kind of stretchy but very healthy.. But it definitely is lacking in the "bushy" department.

Do you have any light leaks in your grow space? Example, exposed light via a humidifier or holes in tent? Light leaks can leak to unexpected flowering cycle! Otherwise the plant looks to be in amazing health, good luck and happy growing!

That plant is flowering. I don't think it's possible for a photoperiod plant to flower unless its getting 12 or more hours of light. Make sure the timer is on for 18 hours and not off for 18 hours. It must be an autoflower.

If you don't want it to keep flowering and grow it bigger still you might want put it to 24-0 for a few weeks and see if you can get it to reveg. It is def in flower now, it'll only make that one main bud if you don't get it stopped. You can go to 12-12 and just do that though.

It started to flower. If it is already flowering in 18/6, there is no need to go 12/12. It will be done quicker that way.