Please keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes only. From the symptoms you describe it is possible that you are having migraine headaches.
When patients present with new headaches, neurologists first examine for secondary causes of headache such as tumors, pseudo-tumor cerebri (increased intracranial pressure), strokes/hemorrhages and vascular malformations. Since you have normal imaging in the past suggests that it is unlikely that you have a secondary cause for your headache. Tumors typically are present for years before they grow large enough to cause headaches and it would be unusual to develop in 14 months.
If the work up for secondary headaches is negative then neurologist examine for primary headache disorders. Migraine headaches are very variable, but can generally involve throbbing headache pain, photophobia (lights bother eyes), phonophobia (sounds bother you), nausea and worsening of headaches with movement. These headaches can also be associated with auras of flashing lights, confusional episodes and stroke like syndromes (weakness, sensory loss on one side of the body). This may explain the your headaches, associated with nausea and memory lapses and the weakness in the right arm/wrist.
Another explaination for the weakness in your right wrist could be carpal tunnel syndrome, since carpal tunnel is very common in pregnant women.
The symptoms of loss of balance, tinnitus (ringing in the ear) and neck pain may indicate a inner ear infection (often caused by a virus).
It is difficult to re-image women who are pregnant due to the possible risk to the unborn baby. (although safety studies are not available for MRI, it is generally used if imaging is needed.)
When your neurologist decides to repeat your MRI of the brain (probably after the baby is born) you might ask him to include a MRV (MRI venogram-examines the veins) and an MRI of your cervical spine.
There are many medications used for migraine headaches, but less are available to you during pregnancy.
I would reccomend that you see a neurologist who has expertise in headaches. Such as a member of a headache center. I hope this has been helpful.
Try not to worry. as far as I'm aware, pregnancy hormones can make things loosen up round your spine (I had a problem with this when pregnant) and the fact that you have neck pain, surely would suggest a possible neck misalignment. Its amazing what symptoms a neck problem can cause - especially head pressure and headaches. I think it would be worth getting your neck checked out by a physical therapist or an osteopath. But of course, I'm no doctor, so it will be interesting to see what the forum Dr says. Good luck.