Yes muscle tremors are a side effect yes and a common one but not painful ones - have you spoken to your doctor? There are things that can be done and perhaps a dosage adjustment might prevent or minimise, I get them myself along with cogwheel rigidity and tremors but I have no pain from mine.
If you google Seroquel XR side effects you can find the information leaflet on the drug
Yes antipsychotics can commonly cause temporary movement disorders such as akathesia (motor restlessness) and dystonia (stiffness). All of this can be treated with a side effect pill. Speak to your psychiatrist about this.
Perhaps increasing your calcium and potassium intake might help?! I use to experience really intense calf cramps that would wake me up at night... they were absolutely excruciating! I now eat a banana, pretty much daily as well as taking calcium and potassium tablets and have found it really helpful. I also take a high dose of vitamin D which might have contributed to my relief too.
Anyway, just a thought!