You were on a very low dose of
xanax for a very short period of time, and it is unlikely that the switching caused what you're feeling. It can be very addicitve, and a person on higher doses does need to taper down, but this not the case for someone who is taking nearly the lowest dose possible for less than a week.
It is however, immediately reacting versus
Lexapro than can take 6 weeks or so before you see the full effects. Lexapro, like most antidepressants, needs to build up in your system before you see noticeable effects.