Amoxicillin and allergic reaction

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.