HRT and Bipolar Disorder

I started taking HRT just after Christmas. Within a couple of weeks I noticed my hair felt better and I felt generally well although I was still having sleeping problems. As time went on, I felt happier and happier. I became quite excited at the smallest of things. One day I felt like lying down in the garden and giggling with happiness! Someone mentioned that I seemed hyper. Then I started getting dizzy spells.
I stopped taking HRT and saw my doctor who asked if I'd ever been treated for bipolar disorder. Then I realised the similarity in effect between the HRT and lithium I took many years ago when I also became very hyper and risk-taking.

Can anyone tell me why this should be? and if there's anything can be done?