Horrible hives - wondering about cause???

I just had the worst attack of hives in my life. This all started slowly Monday night and 24 hours later really started going crazy.

I went to a doctor yesterday and she started me on a medrol dose pack of prednisone and that is thankfully calming things down. The itching, burning, stinging, pain was unbearable yesterday and spreading rapidly but today the spread has stopped and the hives are fading. I hate having to take prednisone but I felt I had no choice.

I am very curious as to what caused the hives though and am curious what people think about this. It just doesn't seem to make sense to me because I think that all 3 of the possible culprits are things I have ingested before with no problems in the past. I am 50 years old now btw, and have seasonal and dust allergies but never anything that manifested itself like this (other than hives from poison sumac about 20 years ago).

The most likely cause of these hives is that I was on a 10 day regimen of 875mg of Amoxicillin 2x a day for a sinus infection. The hives started to appear on the last day of the medication, very slowly, after I had already taken my last pill in the bottle. First I had one or two itchy red marks on my abdomen and thought it was just insect bites. Within 12 hours I had at least a dozen more on other parts of my body so I was starting to think something else was going on. 12 hours later after that there was no doubt I was having some kind of allergic reaction, with the hives everywhere from the top of my head, behind my ears, on my face, my neck, my biceps (the most), my sides, belly, crotch, thighs, and starting to move down to my feet and hands.

Another possible cause is that I ate some "natural" peanut butter on Monday, first time I have ever tried that - I have eaten peanuts & peanut butter many times in the past and never had any kind of reaction though.

Another possible cause is the FLORASTOR (yeast) probiotic that I was taking while on the Amoxicillin to help prevent the runs while on the antibiotic. But again this was something I have taken in the past with no problems.

How likely is that at my age, after years of being exposed to any of these 3 possible allergic triggers, that all of a sudden I am allergic to one of them? Is this a common thing for antibiotics or peanuts?

I really think the FLORASTOR could not have done this and am leaning towards the AMOXICILLIN but....if I was allergic to Amox why didn't I have problems with it in the past? Or why now did I erupt after 10 DAYS on the drug instead of say the first day???

Or maybe there is something in "natural" peanut butter that I don't know about????

Thanks for any opinions.

Thanks for the replies.

I was thinking more on this and remembered that on Monday I had also ingested something else - perhaps for the first time. That was NASACORT AQ - a corticosteroid spray for fighting allergies/sinus congestion. I was given that when diagnosed with sinusitis the previous doctor visit and intending to use it after the antibiotic was done assuming it had killed off anything first. I only used it the one time Monday, and didn't bother the next day because I felt so much better nasal wise already and didn't need it to try another type of therapy (I have been on many different nose sprays in the past 20+ years in fighting allergies, probably tried them all, Nasonex, Rhinocort, I can't even remember them, probably this one too....hadn't had the need to use one in years though).

I used to see an Allergist regularly for my seasonal problems, even got allergy shots at one point, but haven't been back there in almost 10 years. If I could just make an appt there to ask about peanut allergy maybe I will do that but I'm afraid that is not how they work and instead they will insist on complete workup, new physical exams etc, and charge me mucho bucks.

I will have to look up possible side effects or allergic reactions to NasacortAQ too.