But maybe indeed the penicillin can
have something to do with
it. Common adverse drug reactions
in more or less 1% of people associated with use of the penicillins include diarrhea, hypersensitivity, nausea, rash and some more. Also you could be allergic to the drug. Common allergic reactions to penicillin include rashes, hives, itchy eyes, and swollen lips, tongue, or face.
If you think you can rule out allergy and adverse effect then i think it's anxiety related.