Well there is no spell to 'make' you a witch, if you do magick, spells, rituals, etc then you can consider yourself a witch. I don't know how a spell can make you one. What do you mean 'side effects'? A witch is not someone who poisons someone and flies on a broom if that was what you were trying to achieve it cannot be done.
People can post whatever on the internet [and on this aite] and claim whatever without proof of its effectiveness. You probably told yourself these things would happen and your mind choose to hone in on the signs [example, noticing repeating numbers might of been a sign so your mind would only remember the times in the day when it did happen and forget the times it didn't. It might also say cats were a sign so again your brain made note of every cat you saw and ignored other animals so it looked like you only saw cats. It's a normal brain function]
Witches are people, majority who identify as witches are because they follow a pagan religion [wicca, Druid, neo-pagan] and 'witch' is the title they identify as [like how a person following Christianity would call themselves a Christian, a person following witchcraft would call themselves a witch] no spell will make you a witch, its believe, practice, and study that makes you one.