Could intense attachment to and preoccupation with someone, along with their knowledge of your every move from afar, be caused by magic (sihr)?

If love is for the sake of Allah, then it brings the greatest reward and recompense, and anything else is temporary and will vanish, as Allah Almighty says: "Close friends on that Day will be enemies to each other, except for the righteous." If the attachment is due to lust, then the relationship with this person must be severed. As for his knowledge of your affairs, it might be through your colleagues or through jinn if he has a connection with them. If signs of magic appear on you, then there is no harm in performing ruqyah (Islamic exorcism).