Mysterious Allergic Reaction

For those of you who don't know, this is the second time in my entire life that I've had an allergic reaction. The first time was back when I was 12 or so, and it remains a mystery (I had swelling and a rash for an hour or two for several evenings in a row, and we never did discover the cause).
Well, it seems I'm never to know what is ailing me. This second bout of allergies was much worse than the first. It started off two weeks ago, on Tuesday. At first I just had some minor itching, and I just thought my skin was dry, but I broke out in full-blown hives that evening. Jeff was kind enough to go to the store and buy me some Benadryl cream, which helped to relieve the itching.
When I woke up the next morning with slightly swollen hands and minor hives, I decided to go ahead and go to work. Well, my feet ended up swelling up shortly after I got to work, which made walking pretty painful. Fortunately, our girls were going on a field trip that day, so I was able to stay back and off of my feet. I'm so grateful that my feet swelled up early - if I'd gone on the field trip, I would have been miserable!
As it was, I suffered with my swollen feet (and a funny "old woman" shuffle to match) for another two days. That was definately the worst symptom I had.
When I woke up with swollen lips on Thursday in addition to my other symptoms, I decided to stay home from work and go see a doctor. The visit wasn't very helpful - they took a blood sample (which didn't tell us anything) and gave me something a little stronger than Benadryl. I discovered that antihistamines really do make you sleepy. It was weird to sleep for several extra hours when I felt fine, except for the itching and swelling.
I was feeling ok enough to go to work on Friday. It was a teacher work day, so I was able to stay off my feet again, which was nice.
I continued to suffer with my hives over the weekend (mostly in the evenings), until I finally went to the store on Monday and got some Zyrtec. I took some Monday night and was able to enjoy a symptom-free night of slumber. When Tuesday and Wednesday passed with no hives, I was beginning to feel that it was finally behind me, since Zyrtec is supposed to last for 24 hours.
But, alas, the mystery continued. On Wednesday evening the hives came back, so I took another pill. I was fine on Thursday and Friday, and again hoped that I was ok, but then - like clockwork - I started getting itchy again on Friday evening.
So, if this pattern continues, my hives may come back tonight. I wish they would just GO AWAY!!! Or, I wish that we could figure out what was causing them.
For those of you that are wondering, Jeff and I have been racking our brains trying to figure out the source of my allergies. We haven't used new soap, detergent, shampoo, lotion, etc. recently. I also haven't eaten any unusual foods. Over that first weekend we did about 6 loads of laundry and changed our air filter (in case I had developed a dust allergy - and it needed to be changed anyway). We've come up with theories ranging from me being allergic to Jeff (since I had the worst symptoms at night) to hormones to my library book (maybe there was a mysterious substance on it?). Let me tell you, we've thought of practically everything, except what it is, of course.
If my hives do come back tonight, I'm going to call my doctor tomorrow and see if I could take some kind of test to determine what I'm allergic to. So let's just hope they don't come back and I can chalk up this experience to another mysterious allergic reaction that hopefully won't resurface for another decade or two.