Redirect Loop Occurring after install of HTTPS

Hi,
I’m attempting to secure certain pages of my site with the Force SSL Exclusively option on the HTTPS plugin. However when using this option, I get a redirect loop on all non-secure pages, and the supposedly secure pages are not loading as https. Appears to be something to do with the themes php function, but I have no idea how to correct this. Please advise.
Thanks a bunch.
website: http://www.artypartyevents.com