I have IBS with constipation , I also have anxiety
Celexa > ativan...
Ativan is listed to cause constipation; but not very frequently. Most people on ativan don't suffer from constipation unless there are other dietary and life style factor associated. Celexa is known to cause constipation more frequently than ativan.
So to answer your question, though constipation is listed side effect of ativan, I don't think 0.25mg of ativan is directly responsible in your case.
Hope this helps. Let me know if you need clarifications.