Hello,
Thank you for your query at DoctorSpring.com
I understand your concern.
You mentioned in your history, you never had any problems sleeping on sides up before this. Do you have a habit of sleeping on your sides rather than sleeping on your back or prone?
Is there any radiating pain from your neck to your arms, when you lie on your sides? Is there any feeling of numbness?
When you try moving your neck from side to side, do you feel giddiness?
The most possible explanation of your symptoms is due to a pinched or compressed nerve in your neck area. Since nerves pass through very small areas, they are prone for compression. Pinched nerve in the neck, apart from light headedness and uncomfortable sensation, will also lead to numbness along the extremities, with severe pain radiating towards the arms.
Normally it is self resolvable, however if it does not get better, an X-RAY will be required to check the level of compression.
Also is it possible for you to get your BP checked?
Even though it does not seem to be cardiac related issue, an ECG will also be helpful.
Feel free to discuss further,
Regards.

Hello,
Pinched nerve and symptoms arising from it is slightly difficult to treat, however you need to avoid the posture that is causing the discomfort, and a course of NSAIDS will also be helpful to reduce the associated inflammation.
But pinched nerve does not cause heart beat related abnormalities. Since you're already on Toprol for your tachycardia, i will advise you to get an ECG done too. Some patients experience what is known as ' Positional Premature Ventricular Contractions ( PVCs ), for example on turning their head, or on laying on the sides, an abnormal rhythm in the heart is triggered off. These seem to come and go, but if it's persistent a cardiac evaluation with 24 hour holter monitoring is required for making the diagnosis.
Hope this was helpful,
Regards.