What Causes Dizziness While Suffering From Hypothyroidism?

I am muscular and fit, but have RA and am on Enbrel and Synthroid for hypothyroidism. The last two days I have had a hard time waking up. I'm extremely exhausted. I drink two big mugs of coffee and still go back to sleep for a few hours. I want to exercise today, but feel worn out, but haven't done anything to be worn out. I also have been retaining a lot of water the last several days. At first I thought it was because I was on my cycle, but it's winding down. I am retaining so much water right now that I don't feel or look the same.