Your spider is definitely an orb weaver of some sort (Family Araneidae)- those are the spiders that build the stereotypical spiral shaped webs.   This is a very gravid (full of eggs) female which is why she looks so unusual.  I don't know if she is still around and alive but I will bet that she has laid that egg sac by now if she survived.  For some reason she must have gotten knocked out of her daytime hiding place and web.   It's hard to say exactly what genus she is in but I am going to guess that she is a Nuctenea species of some sort.  Below is a weblink to a british Nuctenea, probably a different species than yours but very similar and you can read more about her.

http://www.nicksspiders.com/nicksspiders/nucteneaumbratica.htm