and PS, because my memory was trashed for so long, I set up a chart on my computer with three columns, tracking each day: what I am eating // what meds and supps I am taking // how I feel.
It's very helpful to me to see trends and also to see that I am progressing. If you do that, most docs don't want to read the chart (altho my current doc does, bless her heart), but you can review and make brief notes to report at your next visit to you doc on trends and issues. It beats walking in the door and answering "Gee, I don't remember" to every question, which is what I was doing.
Everybody's Herxes are different, and the same person can have different reactions at different times. At the 4th day on abx, a Herx could well be what you were getting, that timing sounds about right. (Congratulations! Your got a Herx! That means bugs are dying off and releasing stuff into your body. Yay!)
From another point of view, Lyme messes with your endocrine system, so body temp and metabolism can be affected, among other things, causing me to feel strange sometimes.
Also if you are taking any supplements, they can have an impact too. I am very sensitive to meds, and yesterday I took a dose of Astragalus (an herb for 'immune support', recommended by my doc) that was a little bigger dose than usual, and my blood pressure dropped to 70 something over 40 something. (I think the next step down from there is called 'unconscious.') It passed in about 15 or 20 minutes, but Lyme can have the effect of making your body react oddly to lots of things.