Thyroid ploblem or ?

Just took my bp 91/59. I am a 50 yr young woman. Been healthy all my life. Still have my tonsills. Never been one for sitting still and love to get out in the garden and go for walks. I had breast cancer 6 yrs ago. Been doing o.k. until last year. I had muscle cramps in my feet and peeling, splitting nails. I take magnesium and calcium. I still get cramps and my nails are worse than ever. 3 months ago I started to get tired and fall asleep, this hits about noon-2pm every day. I almost fell asleep while driving the other day. I have been getting headaches, I never get these. I'm gaining weight like crazy. 15 lbs. in 3 months. I'm weak, memory problems and at times depressed. My daughter thinks maybe it is a thyroid problem, SIADH. Or if not, what else could it be? Thank you, omah