Yes the symptoms are known to be caused by magnesium
injections and you seem to have tolerated them well as you have mentioned you are not as bad today.
In general this medication does not have that 'sensitivity' issue where you are required to start a lower dose to build up your tolerability.
If symtpoms are are making your routine really disturbed, you feel alarmingly drowsy or excessively anxious everyday then better discuss the matter with your Endocrinologist
as he/she would also check if you are suffering from low calcium levels. Concomitant low calcium levels make if difficult to tolerate magnesium doses given suddenly.
You can also discuss with your doctor if your hypo-magnesemia can be treated with increasing oral intake of magnesium XXXXXXX foods. Sometimes it is not possible as magnesium does not get absorbed well and it has to be taken by injectables only. Given the history of lupus
it is possible you may need only injectables.
In short, wait and see the pattern of your symptoms and if they are bothering your physical condition and daily routine, please inform your doctor ASAP.
I hope you got the answer. Please get back for any related query. Take care.