Magnesium made me feel great, then fatigue set in.

This website is a great resource! I joined to asked a question after searching for an answer and still needing advice.

Just started taking Magnesium about 3 weeks ago. I take a men's one a day multivitamin every day, and have been for years. 

Week 1: I started on 125mg Mag Oxide pill and I felt excellent during the day, I was sure I'd found my cure because I was so clear headed, my anxiety was under control, and I had lots of energy! It was incredible!!

Week 2: I researched Magnesium, decided I was deficient and decided to purchase Natural Calm, Magnesium Citrate. I took one dose in the morning and one before bed. 80mgs x 2 day = 160 Mag Citrate. I began to feel very tired and my energy dropped, I was groggy, sleepy, and brain fog returned. 

I had a Magnesium RBC blood test done during week 2 and my level was at 5.5 mg/dL.

Week 3-4: I went down to 80mg once a day, waited a few days; slight improvement then went to 40mgs once a day before bed. Still no big improvement and all the benefits I had the first week while on the Mag Oxide 125mg have not returned.

I've read about the different absorption rates of the two forms of Magnesium but still can't make heads or tails of why I got such a great improvement in my mental and physical health the first week and now while on Mag Citrate I can't recreate that? Its like the more absorbable Mag form made me worse.

I'm thinking of going back to just the Mag oxide pill once a day at this point. Anyone know what is going on with me.

Male in my 30's