I am Having some mild test taking anxiety with slight nausea / tummy discomfort, I know that antihistamines are used for motion sickness. Could I try?

Non-med options. Try learning relaxation techniques, eg. guided meditation or deep breathing (see YouTube).They lower heart rate & blood pressure, & help with physical & emotional sides of anxiety.Best if practiced regularly. Antihistamines best for anxiety (hydroxyzine is FDA approved, Benadryl (diphenhydramine) =same generation) get into the brain. They unfortunately cut off the "stay awake and alert" signal, not good for exams.
Your Zyrtec (cetirizine)? The medications you already take include an antihistamine - Zyrtec (cetirizine). So, you already know how it affects you. If you feel like you'd do well on the test the way you feel after taking it - you can switch that day to taking it before the test. For some, the drowsiness might make it harder to think clearly even if it made you calm. You may have to decide which is better. Calm or sharp. Study well.