one pill makes you larger and one pill makes you small

I've got a monstrous headache that, from experience, I think can only mean one thing: I am allergic to one of these antibiotics. I'm still very very itchy from what I thought was the sunburn, but I am now beginning to wonder...? The last two antibiotics I was on I turned out to be allergic to, though the hives came the day after the last dose. Friday morning I pop my last pill, so we shall see. Hives are not something I want to deal with, nor is this increased sensitivity to antibiotics.