There are many possible causes!
I read carefully your query and understand your concern.
If can happen to have these symptoms once in a while for example if it was a hot day ,if you haven't drink enough water these days or you didn't sleep well.
It is not concerning if it was just one day,as I said it can happen .
You should rest,eat healthy (mostly green vegetables) ,drink plenty of water and wait some days to see how you will be.
Sometimes a viral infection can cause these symptoms 1-2 days before other typical symptoms appear.
Since they are a lot of causes for these symptoms, in order to evaluate the significance of these it is important to see how they will progress for 2-3 days.
If the symptoms persist (you feel dizzy,sick,tired ) more than 2-3 days you may need a complete evaluation and some tests to exclude common causes for these symptoms including:
.You should run a complete blood count
.If you have low RBC
and low hemoglobin
,you have anemia.
-Oscillations of blood pressure.Measure your BP when you feel dizzy and sick.
-Hypoglicemia or Diabetes
.Run a fasting blood sugar .
If these tests result normal your doctor may order a cardiac check -up to exclude heart conditions.
Hope it was of help.
Let me know if you have any additional question or doubt to clarify ,I will be happy to assist you further.
Wish you all the best!