What can cause a strange anxiety?

Strange anxiety: Most anxiety feels "strange" to the person feeling it. It is uncomfortable, and comes with physical sensations that are uncomfortable too: heart racing, pounding, or even skipping beats; shortness of breath; dry mouth; inner agitation; sweating, etc. Various anxiety disorders can cause this problem; also depression can bring anxiety with it. Other possibilities too. A psychiatrist can help.

Anxiety: Wondering if you meant stranger anxiety, which is separation anxiety. ?

Confusing , indeed: Implicit or procedural memory develops @ 5 mos. If mom brings him a bottle, he knows she'll feed him & knows caretakers vs. Strangers. Hence, stranger anxiety @ 6 mos. Object permanence, retaining a mental image of a person or thing that's out of sight, is @ 8 mos., when he looks for a bottle that he sees mom put under a blanket. Hence, @ 9 mos., cries when mom leaves, normal separation anxiety