。
Ami just left, we must've passed each other.
Literal
Just-now passing-each-other [with-で] Ami-[polite-さん] [subj-が] went-out-[moment-ところ] [copula-です].
入れ違い (literally '(the) mistake of entering') is a set noun for 'missing each other / passing without meeting' — as in when one person leaves just as another arrives. ~たところ '(has just) done X' uses the ところ noun to mark the point or moment of completion. さっき 'just now' is the casual past-temporal adverb. The whole sentence conveys a 'just missed' regret.