Constant exhaustion can be caused by fungal infections. It's one of the possible side effects listed on the medication. Antibiotics and immunosuppressives kill of your good gut flora and, then, the fungal infections have an
open door.
It's something to ask about when you see the doc. Low magnesium is famous for causing migraine headaches, so that is another question. I had candida overgrowth for over 30 years before it was discovered and probiotic supplements with 10 strains of gut flora resolved many symptoms. There are many people on this site who use probiotics to re-establish healthy gut flora levels during and after antibiotic use. I'm sure they can contribute more than I to the overall discussion.