IIII

Looks like it's not getting enough sun

As Manuel has stated it is not getting enough sun all over. It is literally trying to go towards the sun. It may be getting sun thru the window, but it’s not getting any sun on the other side. You could periodically turn the cactus to help the issue, but for optimal growth it should be grown outside during the growing period.

Same cactus as yours, but grown in full sun.

If you do decide to give stronger sun, do it slowly because even cactus burn when not acclimated.

Well sun hits my window during 1 pm so that's half a day vanished