Side effects of low-dosage Xanax?

I've been taking Lexapro (10mg) for about two months now, and it's helped with general anxiety. However, I'm still getting periodic panic attacks, so my psychiatrist prescribed 0.25mg of Xanax to take as needed.

I think this is a relatively low dosage, but I took one last Friday before going out with my boyfriend, and the night went horribly wrong. I couldn't focus, I was extremely drowsy and out of it, and I couldn't hold a conversation.

But I took one today before work and haven't felt any effects at all.

Does anyone else get weird side effects? I can't tell if it was because of the environment (we were in public and it was loud). But the one I took today also seemed to have zero effect at all.