Amoxicillin and doctor who

My Husband has taken this all his life of course like many of us have as a child and teen. He is now 27 and started this on Tuesday for a sinus/upper resp. infection. Evening of day 1 with first dose and day 2 nothing. Day 3 he started itching and didn't know why. Day 4 lips, hands were swollen. Tongue slightly swollen. Hives started on shoulders and inside of thighs that night. Day 5 swelling down after starting benadryl but hives all over body or rash all day off and on. Day 6 back to the doctor who said that he had an allergic reaction and was now allergic to Penicillin.
Strange but it is true that you can be allergic even after taking it for so many years beforehand.

I was prescribed a 10-day regimine of 500 mg capsules, 3 times/day for sinusitis. Day 1 with all 3 capsules, there were no side effects at all. After the 1st dose on day 2, I noticed that I felt hot and weak. I looked under my shirt and my skin looked sunburned. I checked further and found my whole body from neck to toe was bright reddish, like sunburn. I immediately discontinued thinking I must be having an allergic reaction. The next morning, the "sunburn" changed to thousands of little red dots all over. I called my doctor who confirmed I am now allergic to Amoxicillin, even though I used it in the past for probably 10 times with no side effects at all.