I've never had a headache except maybe when I did high doses and didn't eat, rest or hydrate right. Or combined (with things like cannabis and other). Allergies, never heard of them. With research chemicals there is a bigger unknown and also it can be a case of mistaken identity which is why a test for allergic reactions is recommended... so to speak. It's not really an allergy to worry about but you never know with these novel drugs.
With LSD though, I have never heard of someone eating an insanely tiny part of a hit to check it out, I would call that overreacting. Relax, man. Trembling may occur as part of an electric energy feeling in your body but it is not bad, it feels extraordinary and is all part of the experience.
The anxiety itself can be your worst enemy, really. And it can cause intestinal distress and other common anxiety symptoms. Rather than relying on pharmaceutical remedies, have trust and faith that it will be alright. Just ride out the trip, there is no reason to worry. Let yourself go and surrender to it, in the process you will relax deeply and it feels totally right.