Only taking half of a .25 is an incredibly small dose. I don't think withdrawal effects would be very noticeable. And you would start experiencing them in the first 24hours of not taking it. You should still talk with your doctor. The feelings you are having may be the anxiety the xanax was originally prescribed for. When I was taking 1.5mg of xanax a day I noticed withdrawal effects about 8-12 hours after my last dose, its been awhile so I can't remember the exact timing. I take .25 now as needed and I go for a week of taking 1 daily to not taking any for a week depending on my life situation at that particular time and I haven't experienced any withdrawal.
But everyone is different. And going cold turkey off of xanax should never been done if taken regularly. So you should check in with your doc and figure out if its withdrawal or the anxiety.
You really do need to taper off of this medication as you have found out. The best thing to do though is to discuss it with your prescribing doctor so that you can come up with a proper tapering off plan.