Title: Magnesium glycinate started giving me anxiety after long-term use — anyone else experienced this?

I’ve been using Designs for Health Magnesium Glycinate for quite a while. Initially it worked really well for me — helped me relax and seemed good for sleep.
Recently, though, I’ve noticed something strange. Occasionally after taking it I seem to get the opposite effect: increased anxiety/restlessness and sometimes I become more aware of my heartbeat.
I’ve been taking around 300–350 mg of supplemental magnesium, usually at night. I’m wondering whether this could be related to:
The glycine component rather than the magnesium itself
Taking too high a dose for me
Developing a different response after using it for a long time
Electrolyte balance or another nutritional issue
Or simply anxiety occurring coincidentally and me attributing it to the magnesium
I’m thinking of reducing it to around 200 mg and keeping everything else consistent for a while to see whether that makes a difference.
Has anyone else found that magnesium glycinate was calming initially but eventually seemed stimulating or anxiety-provoking?
If so, did lowering the dose help? Did you switch to another form such as magnesium citrate, malate, or threonate? And did you ever figure out whether it was the magnesium, glycine, dose, or something else?
I’d especially appreciate experiences from people who had been taking magnesium glycinate successfully for months or years before this started happening.