Well, high blood sugars can be caused by both stress and illness, so it is possible that your readings have nothing to do with the previous day's activities, other than the fact you had a stressful time and are now feeling ill.
Not sure about the likelihood of a diabetic low when you're not on any meds. Something akin to reactive hypoglycemia??? You'd really have to do a blood test to prove it, because it's not supposed to happen if you're not on insulin or certain types of T2 meds.
Dizziness can be caused by all sorts of things other than hypos....
Other thoughts - are you sure those "sugar biscuits" were only 6g of carb? Seems very low.
While 8.9 is outside normal not diabetic blood sugar range, it's not very high eg not high enough for sugar to pass to urine. I wouldn't expect it to make you feel ill, though I know that some people are very sensitive to even quite low high blood sugars.
If you're not medicating then the only way to deal with high blood sugars is too not eat carbs and/or exercise. So eating a biscuit is probably a bad idea....
Honestly, I know it's tempting to assume that all unwell feelings are caused by diabetes, but non-diabetics get sick too...
Having said all that, I'm T1, so not necessarily the best person to advise. Hopefully some T2s will make some suggestions too.